Electric Car Debate

[dereckbc]

Grab a beam-style torque wrench. Torque a bolt to 100 ft-lbs. Now, apply force on the end of the wrench until it measures 50 ft-lbs. Calculate the amount of horsepower you're producing at 0 RPM with that wrench.

Done it many times you apply pressure, it turns (RPM) and loads a spring with a readout.

 

[dereckbc]

Is anyone here actually saying that electric motors _cannot_ produce torque at zero speed?

-Jon

The answer is yes and no, it is undefined. How would you measure or define the torque without a movement to measure with to begin with?

 

[tallgirl]

Done it many times you apply pressure, it turns (RPM) and loads a spring with a readout.

When you do this, and the wrench stops turning (which is why I said to torque to 100 ft-lbs first ), does the readout continue to display 50, assuming you keep applying the same amount of force? And after the wrench has stopped turning, and the readout still says 50, what's the torque? The answer should be "50". What's the RPM? 0 (the wrench isn't moving). What's the horsepower? 0 (there are no RPMs, so there is no force over a distance per unit time).

 

[winnie]

Is anyone here actually saying that electric motors _cannot_ produce torque at zero speed?

-Jon

The answer is yes and no, it is undefined. How would you measure or define the torque without a movement to measure with to begin with?

On this point I strongly disagree. You most certainly can have torque defined in a stationary system, just like you can have weight defined in a stationary system.

If I stand on a scale, the scale _must_ move slightly to register my weight. But if I stand very still, so that all motion stops, the scale _continues_ to register my weight.

Thus I quite agree that there will be initial motion, and that if you define the question to mean 'no motion, at all, ever', then the answer is undefined. But if you permit the system to be built and to come to equilibrium, you will still have definable forces, even though the system becomes stationary.

It is quite true that when the motor initially applies torque, the system will have to re-adjust to accommodate the different forces. The electric motor shaft will twist a bit, and there will certainly be some amount of motion at your force sensor. This motion can be made arbitrarily small by making the system stiff enough (thicker shaft, sensor that requires less motion to register, etc.)

But once everything has adjusted to equilibrium, the end result will be a shaft that is not spinning, applying torque to some load that is not moving. The electric motor is producing torque at zero speed.

For the purpose of the present discussion, this is in contrast to an internal combustion engine, which simply will not function at zero speed. Because of this, a _clutch_ (or similar mechanism) is required. The ICE spins and applies torque to the clutch. The clutch slips, and applies torque to its output. The output need not be moving at all, but torque is still applied.

-Jon

 

[Bob NH]

You can have torque without speed, but you can't have power without speed.

To get your Corvette to 100 miles per hour (146.667 ft per second), if it weighs 100 slugs (3217 pounds), requires 1955.6 Horsepower-seconds net delivered to the vehicle. And it doesn't matter whether the torque from the engine is 250 ft-pounds or 500 ft-pounds.

1/2 * 100 slugs * 146.667^2 = 1,075,556 ft pounds = 1955.6 Horsepower-seconds

Notice that there is no torque in that equation. Power = Torque x angular velocity and divide-by-zero trickery won't change it.

 

[winnie]

You can have torque without speed, but you can't have power without speed.
[...]

1/2 * 100 slugs * 146.667^2 = 1,075,556 ft pounds = 1955.6 Horsepower-seconds

Notice that there is no torque in that equation. Power = Torque x angular velocity and divide-by-zero trickery won't change it.

On this point you have my absolute agreement. The kinetic energy of the vehicle must be supplied by the engine/motor, in addition to any losses which must also be supplied.

You can express this in units of horsepower seconds, kilowatt hours, newton meters or foot-pounds.

Expressing this in terms of foot-pounds is particularly interesting: the total kinetic energy that must be supplied to the vehicle must be equal to the integral of ( force times distance) applied to the vehicle.

Consider a vehicle accelerating from stop to some 'base speed'. The initial torque at the wheels at zero speed delivers _zero_ power, and thus adds nothing to the final kinetic energy of the vehicle. But the initial torque at the wheels at zero speed determines the _acceleration_ of the vehicle from stop.

I posit (but cannot confirm without detailed analysis) that:

1) An internal combustion engine vehicle, properly geared so that the engine can deliver 200hp to the wheels at 60 mph.

2) An electric vehicle, properly geared so that the motor can deliver 200hp to the wheels at 60mph.

3) Both vehicles with the same variable speed gear box (if one is present).

That the electric vehicle will deliver greater torque to the wheels at zero speed, and will take less time delivering the same net horsepower-seconds.

-Jon

 

[tallgirl]

You can have torque without speed, but you can't have power without speed.

To get your Corvette to 100 miles per hour (146.667 ft per second), if it weighs 100 slugs (3217 pounds), requires 1955.6 Horsepower-seconds net delivered to the vehicle. And it doesn't matter whether the torque from the engine is 250 ft-pounds or 500 ft-pounds.

1/2 * 100 slugs * 146.667^2 = 1,075,556 ft pounds = 1955.6 Horsepower-seconds

Notice that there is no torque in that equation. Power = Torque x angular velocity and divide-by-zero trickery won't change it.

Alright, I've been told that I come across as an arrogant SOB, so I'm going to try and discuss this arrogantly.

The discussion wasn't about peak horsepower, which is what a 140HP rated Honda Civic motor is describing -- its peak horsepower.

Here's my comment --

Some of your requirements, particularly horsepower, are off base. Internal combustion engines and electric motors aren't comparable when it comes to the exciting world of torque. And with internal combustion engines, torque is all about acceleration, not highway cruising.

So, you can get an electric motor which makes big, fat torque numbers at low RPMs (and remember that torque and horsepower are mathematically related by RPMs -- Torque x RPMs / 5252 = HP) where a gasoline engine is either stalled or struggling to overcome internal friction, but won't make the same horsepower, and still have a very peppy little car.

Top speed -- the last time I drove 90MPH was weeks ago. Acceleration is more important than (illegal) top speed, and as I showed a second ago, electric motors can get off the line just fine without all that horsepower.

Jon has confirmed this several times now, including here --

In general: Electric motors produce constant torque over a very wide speed range, right down to _negative_ RPM. ICE engines would stall. Electric motors operate much more reliably in 'overload'. For short duration applications you can safely operate an electric motor at current levels that would _melt_ it on a continuous basis. This can be done repeatedly as long as sufficient cooling time is provided. Try to do the same thing with an ICE and it will explode in very short order. Electric motors tend to be rated at their continuous power output basis, automotive ICEs tend to be rated at their peak power output. (ICEs for other applications will be rated on a continuous basis, however.)

It's this continuous torque property, as well as the ability to produce torque at 0 RPMs, that makes electric motors work in certain aplications. For example, where Jon described it here --

But once everything has adjusted to equilibrium, the end result will be a shaft that is not spinning, applying torque to some load that is not moving. The electric motor is producing torque at zero speed.

For the purpose of the present discussion, this is in contrast to an internal combustion engine, which simply will not function at zero speed. Because of this, a _clutch_ (or similar mechanism) is required. The ICE spins and applies torque to the clutch. The clutch slips, and applies torque to its output. The output need not be moving at all, but torque is still applied.

An electric motor doesn't require a clutch or torque converter to "slip" from a low speed start. If it takes 100lbs force to "start" moving my car from a stop light (static friction), an electric motor can overcome that with exactly however much torque it takes, once gearing is taken into account, to deliver 100lbs force to the wheels. As Jon wrote in the piece I quoted above, a gasoline engine cannot do that -- it requires a clutch of some sort.

To get back to the 1955.6 HP-seconds, yes, absolutely correct. But the amount of horsepower which must be delivered at any given instant in time by the motor isn't determined by the mass of the vehicle OR the velocity. It's determined by the desired rate of accelleration, which George alluded to here --

Bob, I'd tend to agree with Julie's take on it - If a smaller sized electric motor would have the same sensation to the driver as a larger gasoline engine would, I think it's worth noting in the context of the discussion. If a car is unresponsive enough, even the most long-haired yada-yada would not drive it for fear of getting hit in the rear.

I can deliver those 1955.6 HP-seconds over the course of an hour, a minute, or 12 seconds. If I choose an hour, the horsepower required is one value, a minute another value, and 12 seconds it comes out to be something close to 300 rear wheel peak horsepower or so, as I recall. For each of those rates of accelleration there is a corresponding force which must be applied. That 140HP Honda motor from Dereck's original post --

I won?t give all my thoughts away just yet, but if you crunch some numbers, see what you come up with and see if you think it is even remotely possible. Here is a short list of things to consider.
Horsepower to KW. My Honda Civic has a 4-cylinder 140-hp engine
Weight of the vehicle including the fuel source
Cost of electricity. Hint I used $.13 per KWH.
Battery capacity needed.
Weight of the batteries, use any technology you want. Hint; Lithium Ion has the highest density ratio.

-- isn't putting down 140HP except at wide-open-throttle, RPMs at the peak of the band. And that's moving right along. Put that motor in my Corvette and it will reach 100MPH, just a little bit slower. Put an even smaller motor in, and again, it (may -- let's don't get carried away here) will reach 100MPH, just a little bit more slowly. All three hypothetical motors put down the same net total, they just did it over different time periods.

Now, if you look at what Jon wrote, and compare that to my dyno slip --

An 4 pole 60 Hz electric motor rated at 200 hp and 95% efficiency will have a nominal continuous torque rating of about 600 foot pounds. Put this motor on an appropriate drive, and it will produce 600 foot pounds at 0 RPM (power output 0 hp, efficiency 0%).

You see that the 200HP electric motor he described here is producing twice the torque -- 600 ft-lbs -- at 1800 RPM (because it's a 4 pole motor ...) as my ZZ4 motor (likely -- chart doesn't go down that low, but let assume it's constant down to stall speed, which is about 500RPM or so) is at the same 1800 RPM. What that means is that unless I'm driving with the engine at 3600 RPM, where 2:1 gearing would produce 600 ft-lbs torque (and roughly 200HP as well), I don't need even a 200HP motor, much less that thing I've got, to produce the same "experience".

This is what I disagreed with in that first post -- the relevance of a 140HP gasoline motor. If a 200HP electric motor produces twice the torque at 1800RPM (which is 54 miles per hour, 0.7:1 4th gear (OD), 3.55 rear gears, 265/70R15's, I think) as my 250-mumble HP GM ZZ4, why again do I care about what size gasoline motor a car has?

 

[iwire]

The discussion wasn't about peak horsepower, which is what a 140HP rated Honda Civic motor is describing -- its peak horsepower.?

I was under the impression the discussion was about a the practicality of an electric car.

From the opening post.

Basically I take the stance a true electric car will never be practical unless there is a major leap, and I do mean a major leap in battery technology.

You decided to focus in on particular issue in the post which changes nothing about the general question. Bring a high torque motor to the party, it is worth nothing without a convenient source of power.

Alright, I've been told that I come across as an arrogant SOB, so I'm going to try and discuss this arrogantly.

If you want to post what is said in a PM that is your choice, heck cut and paste the entire PM if you want.

 

[dereckbc]

Julie I never said it had to be 140-HP, go back and read. If you do not like the term HP, use KW if that floats your boat. Otherwise IMO just stay out of it as you are not contributing anything positive or productive in the conversation.

 

[tallgirl]

You decided to focus in on particular issue in the post which changes nothing about the general question. Bring a high torque motor to the party, it is worth nothing without a convenient source of power.

I think that before "convenient source of power" can be discussed the size of the motor required has to be determined. Throwing out a number like 140HP doesn't say what's actually needed to operate the vehicle. It only says what the peak horsepower of the motor is.

What is the driver going to do? How fast do they expect to go? And how fast do they expect to go that fast? From those parameters you can work backwards and calculate the peak horsepower for accelleration and the average horsepower required for cruising. Add all those up and you get the total power requirement.

Starting off with a 140HP gasoline motor as a comparision is, in my opinion, going about it the wrong way. That 140HP motor will likely never produce 140HP in a year's time unless the driver sets out to do so intentionally. For sports car drivers that's one thing -- I drive a '79 Corvette and a supercharged Pontiac, I'm going to use all the available power I can find because I'm like that -- but everyone isn't out to buy an electric car that is sporty. And if they are, well, there's one of them --

http://news.com.com/1606-2-6102127.html

 

[iwire]

I was not aware that Dereck challenged us to design this car.

 

[tallgirl]

I was not aware that Dereck challenged us to design this car.

I wasn't aware of it either. He listed what he thought were issues, and I responded. People didn't like some of my statements, and I made arguments in the formal argument sense to support them. Some people didn't like my comments that electric motors make torque at 0 RPMs and a giant discussion ensued. Some people didn't like my Carroll Shelby quote.

FWIW, I think Jon's points about electric motors and how they behave are highly instructive. And I think the last URL I posted shows that sporty electric cars are feasible.

If you have any questions about how electric motors work, refer to Jon. If you have any questions about how sporty electric cars work, watch the video. If you want to talk about small block V8s, home brew or residential wiring, feel free to PM.

 

[George Stolz]

If you want to post what is said in a PM that is your choice, heck cut and paste the entire PM if you want.

Now, to be entirely historically accurate, I believe we said she was an arrogant cocky SOB in the EMF thread out in the open, so cutting and pasting PMs should not be entirely necessary...

(As I duck and run for cover...)

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[LarryFine]

In no particular order, my thoughts:

This reminds me of the watts vs. watt/hours discussion I've had in the past.

Anyone who's ever bored a framing member with an auger knows that torque can exist at 0 RPM. Grab the chuck of your 36v DeWalt and hit the trigger.

One thing I've noticed that pops in and out of the various equations is time. I believe that HP requires time for power to be expended, while torque is an instantaneous value.

If someone asked me how much power my engine makes, I'd have to ask "over what period of time?"

The formula for HP includes torque, revolutions, and minutes. There must be applied torque, movement, and a time period over which the measurement is made.

Ohm's Law depends on voltage and resistance to determine current. While we can juggle the values to find the unknown, amperes will always be the result of the other two.

Likewise, Watt's Law (my name for it, anyway) tells us that power is the product of volts and amperes. Again, we can juggle the numbers to suit the math process, but watts is the result of the other two.

However, for power to actually be expended (used to create motion, heat, light, etc.), we must have a time lapse. The absolutely instantaneous (zero time) power output of anything will always be zero.


(Usual disclaimer: If I'm wrong, kindly disregard this information.)

 

[dereckbc]

Julie,
I did go to your link on the Telsa Roadster quite some time ago when I got interested in this whole thing. From what the manufacture claims, it does appear to sound feasible until you read the specs and crunch the numbers. Something very fishy going on.

Motor
? Type: 3-phase, 4-pole electric motor
? Max net power: 185 kW (248 hp)
? Max rpm: 13,500
? Efficiency: 90% average, 80% at peak power

Performance
? 0-60 mph (0-100 km/h): approximately 4.0 s
? Top speed: 130 mph (210 km/h)
? Range: 250 miles (400 km)
? Fuel efficiency: 200 watt-hours per mile

Battery
? 6,831 Lithium ion battery cells
? About 450 kg
? Full-charge time of three and a half hours
? ~50 kWh capacity
There are a few key numbers to look at, and you can get an idea of the range of this monster. First thing I looked at was the size of ther motor (KW rating 185), and top speed of 130 mph. I then used this number to determine the KWH draw on the batteries based on the claim they tested at 200-watt hours per mile they claim.

So here goes. Use a speed of 65 mph. At 65 mph we can determine the KWH rating or horsepower for kicks to equal 46.25 kwh. How did I come up with that? Simple it must take 185 KW to make this thing go 130 mph. In order to move the same mass at half the speed would be ? of the power to go 130 mph.

Well something stink already because the batteries are rated at 50 KWH, meaning the are spent after one hour travelling at 65 mph for whay distance; 70 or 75 at best. Far short of what the manufacture claims of 250 miles per charge.

So as I scratced my head for a while, I thought OK lets drop down to 30 mph. This gives us 11.5 KWH, which correlates to the 200 watt-hours per mile they claim. Crunch the numbers and you get 4-hours run time for 120 miles at 30 mph. Still way short of the claimed 250 mile.
Only way to get close to 250 miles per charge is at 15 miles per hour. Just as well ride a bike to work, it would be faster.

 

[tallgirl]

Dereck,

I don't know that they claim the motor is at peak power at 130MPH. That's one thing that could explain it.

For example, it could be that the car has a governor which keeps it from exceeding 130MPH, much the way that many sports cars have speed limiters. There are a number of possibilities, including that the power has fallen off by 130MPH and that's just all it will do. In my mind that's far more likely -- the power curve isn't flat, and they say so.

I really don't want a repeat of the last several pages worth, but 46.25kW is 61HP. I'd be surprised if the car took that much power to go 65MPH.

What we can do to sort of sniff-test the numbers is convert the 61HP into linear force. We can take the direct approach, or the not so direct one. I'll do it indirectly, then sanity check with the direct approach.

The tires are 225/45R17. If I remember how to convert that correctly, the sidewall is 45% of 225mm, or 101mm, and the radius is 8.5", or about 215mm. Converting back to inches gives 326mm total / 25.4 or 12.8" (why does that sound wrong? -- someone check my math) for the radius, and 12.8" x PI x 2 = 80.4" for the circumference. Converting that into wheel revolutions per mile is 5280 * 12 / 80.4 = 788 revolutions per mile. Assuming 60MPH (just because that's a mile a minute ...) that's ... 788 RPM and, say, 52HP (61 * (60 / 65) ^ 2).

Plugging those in to get torque gives T = 52 * 5252 / 788 = 346 ft-lbs. Correct that for the wheel radius, 346 * (12 / 12.8) = 324 pounds of force actually at the pavement. The sanity check is 324lbs * 5280ft / 60seconds / 550lb-ft/sec/horsepower = 51.8 horsepower. We could have worked this the other way, obviously, and determined that 52HP is a force of about 324 pounds over a distance of 5280 feet in 60 seconds.

To me, it seems very unlikely that it takes 324lbs-force to move that car down the road at 60MPH. Or even 65MPH. I couldn't find the coefficient of drag numbers, or the projected area, or anything else needed to compute the drag at 60MPH. But to give you a basic idea, 324lbs-force drag translate to 0.13g deceleration just taking your foot off the pedal at 60MPH. Since drag increases by the square of the speed, that would mean the drag would have to be 4 * 324lbs-force at 120MPH or 1,520lbs-force at 130MPH. And that would be a deceleration force of .61g, and that is definitely wrong.

So, without them providing the drag coefficient numbers, I'm 100% certain that thing isn't taking 52HP to go 60MPH.

And I hesitate to say this, because I really don't want a repeat, but what happens with vehicles is that the power is falling off at the same time the drag is coming up. Where they intersect is "top speed". "Maximum horsepower" happened long before the Tesla reached "top speed". And that's what I suspect is most likely happening -- 185kW is the "maximum", it's just not the power being produced at 130MPH.

It looks to me that 125HP is the 13,500RPM point on their power curve (http://www.teslamotors.com/performance/performance.php?js_enabled=1). You can give that a looking at as well and tell me if you agree. That would put the amount of power required at 65MPH to overcome drag closer to 31HP, or 23.1kW. That's still not the range they claim -- 50kWh / 23.1kW = about 2 hours, 2 * 65MPH = 130 miles. There's no regenerative braking on a road trip, so recovery is 0kWh from braking.

(Edited to add ....)

I wasn't going to touch the 200 watt-hours per mile information, but here goes.

200 watt-hours per mile is 12kWh per hour at 60MPH. Right? 200 watt-hours / mile * 60 miles = 12,000 watt-hours per hour, or 12kW at 60MPH. 12kW = 16HP. Nah, I don't think so. Before I poo-poo that completely I'd have to know the drag coefficient.

 

[LarryFine]

Motor
? Type: 3-phase, 4-pole electric motor

Wouldn't a 3-phase motor have at least six poles, or another multiple of three?

 

[winnie]

I don't know enough about the entire picture of the automobile industry to answer all of the points raised while I was out biking So just a few comments:

1) The _power_ necessary to move a car at constant speed is equal to the _speed_ times the _force_ needed to overcome all drag on it. The dominant drag components are aerodynamic drag, wheel drag, and work against gravity if the car is changing altitude. We'll assume a flat road, and ignore gravitational work. Wheel drag is very roughly constant with speed. Aerodynamic drag scales roughly with the _square_ of the speed. Since the total force scales somewhere between the first power and the second power of speed, and since power is equal to speed times force, the _power_ necessary to move the car against drag scales at something between the square and the cube of speed.

For an example discussion, see http://mb-soft.com/public2/car.html, where the claim is made that to move a Corvette at a constant 60mph 26.4hp is needed _at the wheels_. This is 328 watt hours per mile, making the claim of 200 watt hours per mile pretty darn impressive but _not_ 'out of this world'. I don't have the information to confirm either of these claims.

2) In answer to Larry's comment about power over some time period: power can be given an instantaneous value, but only if you accept that _speed_ can be given an instantaneous value. Work is force times distance, therefore power is force times speed. The discussion of speed having meaning in the limit of zero duration goes back to the Greeks If you define speed as (change in position) / (change in time), this has meaning as (change in time) _approaches_ zero. This can be used to assign a value of power (or speed) to a particular instant in time. See the concept of 'Limit' in an introductory calculus text.

3) In electric motors, the number of magnetic poles developed is separate from the number of electrical phases. The number of poles is simply the number of N and S poles present in the stator magnetic field. Each pole will generally span several stator slots. The different phase windings will usually be divided evenly between the various slots, and you will usually see at least 1 slot of each phase per pole, usually several.

A three phase, 4 pole motor would most likely have some multiple of 12 _slots_ for windings, so that each phase is present in each pole. I am guessing (given the scale of their motor) somewhere between 36 and 60 slots.

-Jon

 

[dereckbc]

This is 328 watt hours per mile, making the claim of 200 watt hours per mile pretty darn impressive but _not_ 'out of this world'. I don't have the information to confirm either of these claims.-Jon

Jon good comments. I do not know the total weight of a Corvette with the engine and full fuel load, but the batteries alone in the Telsa are 1000-pounds (450 KG) and I am clueless what the total weight and the electric motor weigh. My educated guess is the Telsa weighs more than a Corvette, and if that is true, no way can it use 200 watt-hours per mile @ 60 mph.

Other unknowns are things like heat, AC, lights, power steering, brakes, etc. All these things would require power that so far has not been taken into account.

 

[tallgirl]

1) The _power_ necessary to move a car at constant speed is equal to the _speed_ times the _force_ needed to overcome all drag on it. The dominant drag components are aerodynamic drag, wheel drag, and work against gravity if the car is changing altitude. We'll assume a flat road, and ignore gravitational work. Wheel drag is very roughly constant with speed. Aerodynamic drag scales roughly with the _square_ of the speed. Since the total force scales somewhere between the first power and the second power of speed, and since power is equal to speed times force, the _power_ necessary to move the car against drag scales at something between the square and the cube of speed.

For an example discussion, see http://mb-soft.com/public2/car.html, where the claim is made that to move a Corvette at a constant 60mph 26.4hp is needed _at the wheels_. This is 328 watt hours per mile, making the claim of 200 watt hours per mile pretty darn impressive but _not_ 'out of this world'. I don't have the information to confirm either of these claims.

Agreed on the 3rd order effects on power. Using a 2nd order approximation, which Dereck used, gives a conservative approximation -- the power requirements can't be more than what he calculated, or less than what a 3rd order approximation would give. The correct answer is going to be between a 2nd and 3rd order value, depending on how dominant aerodynamic drag is in the overall force equation at a given speed. If the 2nd order terms (wheel drag) dominate, the 2nd order value is more correct, if the 3rd order terms (aerodynamic drag) dominate, the 3rd order values is more correct. Regardless of which is correct, the forces acting to slow the vehicle when power is removed, are what they are and can be used to sanity check or experimentally verify any result.

To get back to the Tesla example, assuming peak power and top speed are the same (which they aren't for the reasons I gave in my response to Dereck), using a 2nd order approximation yields his 61HP for 65MPH (1/4 of 248 peak horsepower). Using a 3rd order approximation, where aerodynamic drag completely dominates the force equation, yields 31HP for 65MPH (1/8 of 248HP).

One thing I overlooked (d'oh) is that Tesla provides the final gear ratio for the vehicle, and it's given as 7.4:1. Getting back to my comment that top speed is where motor power output falls to meet the rising drag, and Tesla's ("over") 130MPH top speed, and now plugging in the final gear ratio of 7.4:1, 130MPH is 190 ft / sec, or 1701 RPM at the rear wheel, or approximately 12,500 motor RPM. This allows a slightly different set of approximations (math not shown ...) of 150HP for 130MPH and somewhere between 18.75 (1/8th) and 37.5 (1/4th) horsepower for 65MPH.

Since both values are greater than the 16HP that 200 watt-hours per mile at 60MPH would require, I think Dereck's suspicions that the vehicle's 250 mile range isn't given for a 65MPH cruise are right.

Tesla does provide the answer to that question, however, and they stay that the 250 mile range is the "EPA highway milage". Using my superior Google'ing skills (heh), I went and checked out the EPA highway testing procedures, and they provide the information about average speed, which is given as 48MPH. Going back to the 150HP @ 130MPH assumption, 48MPH is 0.37 of 130MPH, and 2nd and 3rd order factors are 0.14 and 0.05, respectively. Multiplying both times 150HP gives 21 and 7.5 horsepower, respectively. Since the 16HP from the 200 watt-hours per mile falls between those two figures, I think Tesla's milage claims are likely correct. Just not correct if one wants to drive 65MPH ...