Example, 31" tires, 4.11 gears, no overdrive @ 65mph engine turns 2896 rpm.
31" tires, 4.11 gears, 5th gear with .83 ratio @ 65 mph engine turns 2403 rpm. About perfect for a stovebolt 6 cyl.
You need to consider tire diameter along with final transmission ratio along with rear end ratio to calculate rpm at a given speed. Also decide what your target cruise speed and preferred rpm is.
This is one of many online calculators. Once you've ran the numbers then it's time to look for the perfect T5.
Optional 5th gear ratios: .83, .76, .72 are the common ones.
