Originally Posted by Mike B
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.html

Using 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 smile

Mike B thank you so much sir.

Just confirmed it's a two ton 6100
(unless it was a typo ment to say 6400?)
Sticker says 16,000 GVW

I belive you are correct needing an Allison Truck tranny
Which is an exciting new realm for me to look into

Definitely want something around 600 Hp for an engine

Suggestions on what Model Allison Trans to start searching
for? I'm also interested to hear motor Suggestions

Out with the new, in with the old.