There are lots of on-line RPM/Speed calculators that will do the math for you, here's one example...
https://purperformance.com/p-29669-rpm-calculator.htmlUsing this calculator I plugged in
MPH = 60
Tire Diameter = 36"
Rear Ratio = 6.17
Trans Ratio = .75
and got about 2600 RPM
The 4L80E has the following gear ratios,
1: 2.482.
2: 1.482.
3: 1.00. (direct)
4: 0.750. (OD)
R: 2.077.
Your existing SM420 has the following ratios,
1: 7.0
2: 3.6
3: 1.7
4: 1 (direct)
If you ever plan to load your truck up, the 4L80E might not have a low enough 1st gear to get you moving...something to think about and you'll need a lot more HP/Torque to do it. You may need to think about using an Allison Truck Transmission.
Mike B
