Here's one for the engineers! While filling the car today, I pondered what the effect would be if every driver only filled their tank 1/2 way instead of full. This would mean each car would be relatively lighter over the road. Now, this is a difficult calculation, because first someone has to figure out the effect of weight on MPG. Then, they have to figure out how this changes as the car eats gas EITHER from 1/2 to empty, or from full to empty. But the average might be that the car was 50 lbs lighter during it's daily travels?