You can calculate it here
http://www.onlineconversion.com/bigger_tires.htm

With 6.5 gears, tire diameter of 38" (a guess), and engine speed of 2,500 rpm you get 43.5 MPH.
Take it up to 3,000 rpm and you get 52 MPH

You can play with the combinations and adjust your tire size to what they actually measure.

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
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-