The Stovebolt Site Forums The Shop Area Driveline Differential gearing 3:38.1 ??.??

Use calculator at the bottom of this page.
http://www.csgnetwork.com/multirpmcalc.html

Only 4 variables.
Just change one at time.

3.90 R&P ratio (your truck)
27" tires
60 mph
RPM = 2912

3.38 R&P ratio
27" tires
60 mph
RPM = 2554

On my 1954 1/2 ton trucks, I use 31" diameter radial truck tires on original wheels and with 3.55 R&P in original rear end.
3.55 R&P ratio
31" tires
60 mph
RPM = 2309

I like it. When I have a few passengers I use 1st gear in my SM420.