Here's one good calculator
http://www.onlineconversion.com/bigger_tires.htm

Just for guessing and illustration purposes:
3000 RPM
30" tire
4.10 axle ratio
1.0 transmission ratio
= 65 MPH

•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-