[Phantom]

While you will have a little more weight ~6lbs per gal if mostly highway driving you should not notice it, but think of it this way every extra time you stop to get fuel you are slowing down and driving at parking lot speeds getting bad MPGs at that time and them restarting and pulling out doing the same and adding idle time.

That time and gas used in each stop assuming it takes 1 min extra of the car running to get gas and that you get 40mpg at 40mph you could drive one extra mile per fill or after 40 fills save one gal on gas.