[BrianG]

I have no idea why manufacturers put a current value on their motors, especially for car use. There is absolutely no way to get a constant current draw on a car motor; you'd have to do no acceleration/braking, and keep a constant speed with no variances in terrain, and do no turning.

Wattage rating is little better. Cars pull a lot of high current bursts, with much lower quiescent currents in between. You'd have to use something like an eagletree device to get average amperage and current (therefore power) and use that as a rough way to make sure your average power requirements do not exceed the motor's constant power rating. And even then, a motor's power rating can change with ambient air temperature.