A 57 1/2 ton pickup should have 3.90 gears in it now. That was std gears for 3100 seres trucks. You will be happier going to a five speed OD trans than going to a 3 speed. A three speed or 4 speed or your current granny low 4 speed all are 1:1 in high gear. A 3.55 ratio rear will help but you are still going to be turning more revs than you want at Atlanta highway speeds. Leave in the 3.90 rear and go to a 5 speed overdrive then play with your tire size if you need to to get the right crusin speed. With the 3.90 gears you will have good 3rd and 4th gear acceleration for that merge lane.
Gael
