You can calculate the improvement here:
http://www.onlineconversion.com/bigger_tires.htm

To find out if you can get faster gears for any rear axle you first need to know what make and model axle it is, and in some cases what year. Then go to any gear shop, like Randy's Ring & Pinion or a bunch of others, and see what they offer for that model axle.

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-