Good point. We get shocked by static when we come into contact with an object of potential well above us or well below us, right? Specifically, the threshold of feeling this charge is about 3000 to 3500 volts, depending on a number of factors. If we see and feel the spark, that's noteworthy. If we hear it, even moreso. If we smell it, we really got zapped. If we feel the hair on our body raise and then discharge rapidly, we say "#$%*er!"
So the tires can be presumed to be an insulator, but our bodies are very conductive with respect to the seats we are sitting in and the carpet/floor mats we rest our feet on. We are sometimes the source of charge as our clothes tribocharge (contact and separate) or even rub across the upholstry.
But sometimes we may touch something that's at a higher potential than us. The grounding in a vessel like an aircraft, ship, or car, is the chassis itself. If we're bonded to that chassis, we float along with it and never get shocked... until we get disconnected with it and touch it again or touch some other reference... aka the ground or earth?
I think we are looking at two distinct sources of charge potential here; the car's chassis and our bodies. We can soft ground our bodies to the chassis of the vehicle by wearing cotton clothing or even ESD smocks and footwear... hypothetically.
But what about the chassis to the ground or earth? Airplanes use static wicks to dissipate charges off the chassis or grounding system as they fly through the air. What if a car had a conductive carbon rope that drug (dragged?) across the road as it moved across the road surface? Would the asphalt/pavement be at a low enough resistance to earth to dissipate that charge? Would there be enough minerals and water content to do this? I know that a porous concrete slab can have enough water content in it in the non-winter months to be in the high static-dissipative range or below 1.0E9 ohms with respect to its surface (Point to Point or Ptt Point top to top) or from Point to Ground or RTG.
The problem with the higher resistance materials is that they don't easily enable charges to flow though them quickly enough or they don't move at all until something intervenes. Then we have to deal with charge fields and induction. I can induce a charge on an adjacent item if I merely discharge to a conductor rapidly in its presence.
Anywho, it usually takes layers of protection to best solve some of the ESD events that occur. Keep in mind that we only need to discharge things to below about 3000 volts to be effective for a perceived static shock.
Add: When I was talking about equipotential ground, this is what a field service guy does. When working on a computer, let's say, he unplugs it from line-thus no earth connection. But he/she sets the UUT on a static dissipative mat, stands/sits on the mat himself, aligns his tools on the mat, hooks the mat to the chassis of the UUT, then plugs his wrist strap to the chassis, now you have equipotential bonding.
