Tire size vs. Gear ratio

[Eagle]

One of the most frequently asked questions in the Jeep world is "What gears should I run with [___] tires?" There are a lot of quick calculators for this on the Internet, but they often aren't very accurate. The reason is that they are based on total (or nominal) tire diameter, and that ignores how tires work.

 

First, the actual size often isn't the nominal size. This is more true for the off-road tire sizes in the format such as 31x10.50-15. That tire is nominally 31 inches in diameter but, if you start researching tires in that size, most actually are closer to 30.5 inches in unloaded diameter. When you put tires on a vehicle and then let the vehicle down to put weight on the tires, the sidewalls squash down and out. The further lowers the centerline of the axle relative to the road surface. There was a time when most tire specs included a number for the actual rolling radius, but those days seem to be long gone. Back then, most manufacturers also gacve us a number for revolutions per mile, which is the most useful of all numbers for calculating engine RPM vs. road speed. When I look at tires today, it seems the manufacturers are competing to see who can provide the least useful information to prospective buyers.

 

Many years ago, when such information was still available, I created a spreadsheet using as much actual manufacturers' data as possible to get a better, more accurate look at the relationship between tire size and engine RPM at various road speeds. I have uploaded that spreadsheet to Google Sheets to make it available to members of the Comanche Club. Here's the link:

 

https://docs.google.com/spreadsheets/d/1pHFuhGgj6dQDfzyfFJH5z7NCDLW2KX3ABQgAJr3lBvM/edit?usp=sharing

 

Note: I'm not sure if this file allows anyone viewing it to make edits. If it does, please DO NOT make any changes. If you want to muck around in it, make a copy and then experiment with your copy. Don't screw it up for everyone else.