A simple idea, perhaps (and for all I know someone's already done this), but what about a "place on ground" script that would calculate the lowest point of a model and place that at Y=0?

Better still, calculate the two lowest points—such as two wheels that are not quite in the same plane—and rotate the whole model so that they both rest at Y=0?

Better still, calculate the two lowest points—such as two wheels that are not quite in the same plane—and rotate the whole model so that they both rest at Y=0?