I'm sure moving less weight around will save some fuel. However, when you figure that the car, passengers and fuel weigh over 4000 pounds, 100 pounds is not a large difference. One of the joys of driving a hybrid is going 600+ miles between fill-ups. I'd hate to go 200 miles just to put five gallons in the tank every four or five days.

Also, I think that the savings would be small enough that any difference in mileage would be difficult to measure due to the normal differences in mileage experienced.