First off 40-45 mph tops for a 66 model C60 sounds slow to me... do you have a two speed rear stuck in low? What is your current rear axle gear ratio?

Here's a calculator to figure speeds and RPM
http://www.onlineconversion.com/bigger_tires.htm
Your 9R22.5 tires measure about 38" tall. Plug the ratio for the auxiliary transmission in the box that says "TransCas Ratio:"

Not all auxiliary transmissions have an OD gear, some are direct and only lower ones. If it is OD there are different OD ratios available depending on the model number 0.81, 0.84, , 0.75, and 0.73 are common ones.
Here are some model numbers and ratios
http://rides.webshots.com/photo/2499436910080251109KaJEMa
http://rides.webshots.com/photo/2623210880080251109VGJOGn

An Overdrive transmission works by turning the output drive shaft faster than the input.... the output is "overdriven". So for a given engine speed you can go faster in OD than direct because the driveshaft and therefore tires turn faster.

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-