Here's a gear calculator
http://www.onlineconversion.com/bigger_tires.htm
For double OD just put the second one in the "TransCas Ratio:" spot.

Another option that's a whole lot cheaper than a GV Overdrive is swapping the existing Auxiliary or just the gears inside for one with 0.73 ratio.

Also what about axle gears, do you have any faster choices there?

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-