When I fill the car from from empty it's about 16 gallons. So from full to 1/4 tank is 12 gallons. Gas weighs about 6 pounds a gallon. 12 gallons x 6 lbs/gal = ~70 lbs. I don't notice the weight difference from full to a 1/4 tank, in this car. However, I have no doubt that a lighter M3 is faster, more agile, and has better fuel economy.
Has anyone done a 1/4 mile run with a full tank vs an empty tank back to back?