Isn’t something done on the Sabbath forbidden even to someone for whom it wasn’t done, until after the Sabbath ends?

That isn’t a Sabbath violation-product. There’s no object here, or anything else, that was produced on the Sabbath. You’re simply on the moving car.

But what about causing additional combustion because of the extra weight of your body?

Seems insignificant to me. It isn’t even indirect causation, but rather unintentional involvement, and also something that isn’t visible to the eye.