This tech tip will be helpful:
http://www.stovebolt.com/techtips/calculate_rpm_tire_axle.htm

I like the first link for the gear calculator http://www.onlineconversion.com/bigger_tires.htm

On the last one, lets assume the following, until you correct the figures you know are wrong.
65 mph
Tires are 8.25X20, so about 38" OD
5.14 rear ratio (what is it really?)
The above figures give you 2,900 rpm at 65 mph is that suitable for your engine?
Or if you had say a 0.80 OD ratio, you then get 2,363 rpm, that would be OK, perhaps a little slow.

This 5 speed from a 61 Chevy you mention, what model is it, is it overdrive?? If it is not OD, then it won't help you go any faster than the 4 speed you already have.

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-