I run 3.90's and I didn't see any big drop in MPG and I drive my car 60 miles to and from work everyday. I run full slicks on a built 31 rearend in my 01 GT. I have never had a problem with my 3.90's. Cars run mid 12's and puts down a 1.5 60 foot time lifting the front tire. Hope this helps.
I lost a little mpg with my 3.73's. Mostly in the 65-75mph range because your like right at the powerband. The best is have a nice 6th gear, with tall gears, that is the best.
I put 4.10s in my car then had a street tune to correct my speedometer...I noticed no change in my gas milage...I do the exact same driving every week..Half highway and half city...so maybe it fell in one and went up in the other...I would never go back to any other gear
Going to 3.73s from the 3.31s lost me about 10% MPG on the Highway. What happens in the city is up to you. If you romp on them it will be lower than with the 3.31s. If your easy on them, you will get a little higher.