Will 3.90 gears make a hugh difference in gas milege on the highway?

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You will lose a little on the highway, and in the city. Nothing drastic. I would go with the Ford Ring and Pinion, and either a 3.73/4.10.
 
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 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.
 
im runnin a 3.90 on my 03gt drove her out to so. cal and got 27mpg but i drive like and old lady

typical highway is about 20-24ish i love them. came from a 3.73 in a fox body, i think they drive better. would be better with a t56 though