Hello from the uk
Ive got my 44 1/2 ton running well now, been through the brakes and suspension, is lowered about 4" all round and has headers and dual system otherwise all original. Has a 59ab motor
I have 6.50 16 tyres all round, I can just about keep up with lorries(sorry trucks) on motorways(sorry highways) that is 56mph. Wondering what tyres others are running maybe 7.50 on the rear would help
I had the stock 3 spd box rebuilt recently (not cheap) on reflection a t5 conversion might have been a better idea
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There are charts online that tell you rpms at 60 mph. You plug in tire diameter and axle ratio etc. Without looking it up, your 6.50s might be about 29-30" dia and the 7.50s maybe 31-32" It will help to some degree.

 

Yes 7.50s would get you down the motorway without the engine being wound so tightly. The rear rims should be 4" wide or more, but that is probably what you have.
Waiting for Stu to chime in on this.
Interactive RPM and MPH Computator Program