There are also little gear change conversion boxes to put in line with the speedometer cable.
A speedometer shop will be a lot of help here.

Just for reference 8R19.5" tires are about 32-33" tall, better yet find the manufacturers specs for revolutions per mile for the exact tires you'll use.
You'll also need to know your rear axle ratio.

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-