Keep in mind, it doesn't make sense to spend $3000 to get better mileage when it'll take you 30 years to make your money back. Might make more sense to go shopping for an econo-bucket at that rate - just a thought.
I measure my fuel usage in Smiles per gallon ... I average at least 30-40 Seriously, BigRed with 3.31's cruising on the highway gets about 13mpg. with the 3.08's I got a little over 1 mpg more. here's where it gets interesting - when the car was bone stock 92,xxx miles, with the 400" bigblock (hi comp - 10.25:1), th400, and 2.93 gears I could get 18-19mpg continuously. Weight is just over 4000# With another 69, a skylark 46K miles) with low comp 350-2, th350, and unknown rear (prolly 3.23) and perfectly tuned the best I could get was just over 15. Weight was just over 3700# So, BigRed with about 2.5 TIMES the hp/tq and 300# heavier was getting about the same mileage with the 3.08 gears.
