Originally Posted by panman
...I put a nice tach in the cab so I can check my rpm... What I need to know is what the true mph would be at 2000--3000 rpm...

This is 2014
Have a smart phone? If not you certianly know someone that does.
Download a speedometer app, try it.

For a gear calculator I like this one
http://www.onlineconversion.com/bigger_tires.htm
For a pretty accurate tire diameter look up your actual tire specs from the manufacturer, see the effective loaded diameter, or revolutions per mile and calculate back to that diameter. Or put a mark on the floor and your tire, roll the truck forward one tire revolution, measure that length on the floor (tire circumference), divide by 3.1415 to get diameter.
I would not trust very much any "diameter" some site or rule of thumb suggest based on the generic sidewall markings, there will be variations.


Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-