The 48 is not yet on the road, well it is licensed and I've driven it around but nothing more on it yet than a cab with doors.
It also only has 4.56 gears at the moment though I have a set of 3.54 to swap in. It should be comfortable at 80 in Overdrive if I wanted to try it, or 65-70 in direct.
Calculate RPM and speeds given tire size and axle ratio here
http://www.onlineconversion.com/bigger_tires.htm

Just because the calculator says you can doesn't make it so; you may not have enough engine to pull the truck along and push the wind out of the way at high speeds.

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-