the transmission has almost nothing to do with mileage unless you do mostly highway driving, in which case an overdrive trans might improve it enough that you'd recover the cost of the swap eventually, if you own the truck long enough

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature we find it attached to the rest of the world" John Muir
"When we tug a single thing on an old truck, we find it falls off" me
Some TF series details & TF heater pics & Rust-a-holics Unanimous parking lot