In your situation, I think an overdrive would be a mistake unless it was a 0.86 OD gear. However a 14% drop in RPMs will not save you much gas.
I've done a lot of numbers matching for OD transmissions because I like T5s. A 0.73 OD gear is a great match for a 3.73 rear axle.
A 0.76 OD is good for a 3.42 rear axle.
Like bartamos has pointed out, the savings will not be that good.
I must ask this . . . How do you like the performance of the 3.08 rear axle? Specifically, I'm curious if the truck seems sluggish in the lower gears. If I recall, the SM420 has a very low first gear. I would guess the 3.08 axle makes it feel less like a granny gear.
Last edited by Lugnutz; 10/03/2018 2:59 AM.
