I have a '47 3800 1 ton. I swapped out the 5.14 rear end with a 3.90. Got it from Randy's Ring and Pinion 800 292 1031. It is now mathmatically possible for the old truck to do 75 mph. I had it out on the highway and kept up with traffic, but the speedo doesn't register above about 53 mph.

Wilber

Don't read the directions, that'd be too easy.