Great Question-
Autos were treated like terrain that moved- and our philosophy was if we thought too much about it then it would be a bother rather than a help tell the story of the game.
Overland movement was 5d6, ignoring the lowest d6, meaning characters could get a solid 14" of move most turns. Max move on roads. If the car didn't move more than 8" non driving characters could fire once with a penalty. Autos couldn't move over any terrain (its the 1920's)- and could be disabled by most "monsterous" critters if close combat was lost. Provided light cover to occupants.
Basicaly there was no good reason to stay in them once you reached your objective, it rewarded roads and gave this awesome sense of impending doom as you drive to your destination...