Ed, you are a Prince. So the tire circumference is in the 116 in. range, what is the second figure, the 500 something ? The tires on this vehicle are actually 235 / 80R 22.5

I have signed up with a Google account which gives access to some very useful online applications for ordinary use. I can view and modify my documents and spreadsheets no matter what computer I am using, or what my location may be. I find this more than convenient at times. These can also be shared with anyone else under the cloud, just like putting it up on your website I suppose.

The number right after the diameter is the calculated revs per mile that the tire will rotate, the number after that is the number of revs per mile that the drive shaft will make using a 4.1 ratio. The Allison MD3060 4th gear is 1:1 ratio, 5th is 0.75:1 and 6th is 0.65:1. It is a double overdrive unit.

Sorry, I should have posted all the parameters I used. As I mentioned, I used the diameter from Goodyear but they didn't list the "loaded radius" Michelin does and they also list the estimated revs per mile which is slightly higher then the number I calculated and presented here.

Bridgestone lists their 8R19.5 at 613 revs per mile so I use that.

I am looking at replacing our 542 with a 2000 allison and 5th gear in that box is .71 with 6th .61. Probably never use 6th because with a 3.73 rear end that would give me 65 at 1500 RPM. But 5th gear gives me 65 at 1800. Still a little low on the RPM but great for flat land running.

This is the formula for engine rpm to mph

MPH = engine RPM x 60 / (rear end ratio x overdrive ratio) x tire RPMile