weight doesn't really matter. Trains weigh a FUCKLOAD but are the most efficient means of travel simply because they don't stop once they start rolling, and are shaped such that they're not affected much by wind resistance.
Cars go fast enough that air resistance is a much greater contributing factor to their efficiency than weight is. In general cars are the problem regardless of their efficiency, because they're always going to be the worst choice for moving anything: they're useful because of their convenience.
So, since we can't get rid of cars, the best choice is to make their impact on our carbon problem as lessened as possible, and the best way to do that is to stop burning things to make them move.