Every book on electrical theory that I've read states the current flows from negative to positive in a dc circuit.
Why then are the battery terminals in a car hooked up with black (neg) to the chassis/ground and the red (pos) to the loads. In automotive the current, as I understand it, leaves the positive terminal, flows through the loads, to the chassis, then back to the positive terminal.
Isn't this backwards from how were taught?
The current vs electrons issue is an artifact of history.
Before we knew whether it was positive or negative particles that carried the current, we made a guess. And we happened to be wrong. Benjamin Franklin set the sign convention that the charge left on the glass by the silk was the "positive" charge, and the charge left on the silk is the "negative" charge. He thought that the glass accumulated charge from the silk, and the silk lost charge to the glass. He didn't have scales accurate enough to weigh the difference.
JJ Thompson discovered that there was something special about the negative, with the photoelectric effect. Two electrodes in a vacuum with a voltage across them. Shine high enough frequency light on the positive and nothing happens. Shine it on the negative, and the circuit is completed when electrons are ejected to the other electrode.
When we learned that the electrons were carrying current, we didn't revise the signs on every diagram. We left them the way they've always been written.
This is why "current" flows from positive to negative, while the actual electrons are flowing from negative to positive. Current considers the hypothetical case that it is POSITIVE charges flowing, and it will create the same field effects as the negative charges that actually flow.
And remember, it doesn't have to be electrons that flow. In metals, it usually is electrons. Because electrons have A LOT LESS MASS than any other part of the atom, and electrons aren't bound by nuclear forces. So electrons carry current in metals. But could a positive charge in a completely different kind of circuit. In the human brain, it is positive sodium ions.
In Alternate History, Benjamin Franklin could've not made his "mistake", and called the charge left on the glass by the silk negative. JJ Thompson would discover that the positive electrode would be special in the photoelectric effect, and current and electrons would flow in the same direction. However, this would make us complacent to think that current is only carried by electrons. It wouldn't make us confront the problem head-on, of what exactly is current?
Current is indeed the flow of charges. And its direction is defined as the direction that hypothetical positive charges would be flowing, which would create the same electric/magnetic field effects as the charges which actually flow.
In the automotive application, it is ARBITRARY which terminal you connect to the chassis. The reason you do this, is to make a voltage reference. No current should flow on the chassis. It should flow on the wires. From the source and back to the source. The reason we bond one terminal to the chassis at exclusively one point, is such that in the event of a wire faulting to the chassis, the chassis can carry current back to the source.