OK then, double overdrive can make up for slow axle gears, although may not be the best solution to speed it up just to slow it down again and the friction losses that come with that.

What model auxiliary transmission? I may be able to tell you the ratios if you have the model number.

If a 6BT then I most often hear that 1,800 rpm or there abouts is a good highway cruising speed for fuel economy and engine life.

If a usable and fast work truck is your goal the original axles may not be the best idea.
Might not be a problem, but newer axles with better brakes, available gear ratios, and easy to find/cheap replacement parts may make more sense, they did/do to me.

Stop by www.4btswaps.com
If you haven't already.

Grigg

Last edited by Grigg; 05/27/2010 6:47 AM.
•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-