Here's a gear calculator to help you figure speeds and RPM.
http://www.onlineconversion.com/bigger_tires.htm

Important to have for towing are gears low enough you can get the load moving.
As well as your direct drive gear, axle gears, and tire size such that you get a comfortable speed while towing with the appropriate engine RPM (higher than cruising rpm when empty but not wound out to tight)
And if you don't have enough to think about already consider a transmission with a close 4th and 5th gear such that they are both useable. Not much more frustrating when towing than to be stuck in a low gear because the next higher is so far away you can't get to it.

HP also plays a factor, the right gears for a 235 with it's somewhat limited HP will be slow for an engine with significantly more HP.

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-