Original post by: Jeffrey Szarley ,
3.42's are a little tall. However, if you're going to run 55-58, the 13 speed is a huge advantage running in 12th gear. If you're going to run 60 or above, 13th gear will work great also. You'll have to experiment running both gears throughout the country. The LP 22.5's are a great advantage in both rolling resistance and helping offset the taller gear ratio. I think these specs are a great place to start. Pittsburgh Power is having great results with "repairing" the EGR issues on that 14 Liter Detroit also. If you purchase this truck, maybe budget in the Pittsburgh Power EGR "repair" and you can possibly have an excellent truck in the long run. At 55mph in 12th gear, with a 275 80 R22.5, and those specs in 12th gear, you should be turning about 1340RPM's which is ideal for that Detroit. Drive the truck first to verify these specs because sometimes gear ratios are changed from the original specs. Good luck to you driver.