Slick 60's

My dad just got 3.13s to run in the Slick I sold him. Will this do any thing for improved gas milage, or maybe just make it not Rev so high? I think the 64 has 3:89s stock in the 9" currently. I swapped in a 302 non HO and C4 from a stang with a shortened DS. Thanks in advance for the input.

Probably will get a bit better mileage, but not a sure thing. I have had a few cars and trucks that had overdrives, and half of them got equal or better mileage in Direct.
One change, though, even if the mileage remains the same, your best economy speed will increase with the taller gears.