Originally Posted by acadianbob
I have an Excel spreadsheet that calculates the circumference of a tire. All you have to do is enter the size. I use it when I am changing size/rim on a vehicle to assess the difference in circumference. For example, my VW has 17" summer tires and 16" winter tires. the winter tires are narrower but have a higher profile so the circumference is the same. If anyone wants a copy of this spreadsheet, PM me with your email address and I'll send it to you.
Dave, I agree with Jeeper that 17 or 18 inch tires are much better for load than a 20. Depending on the size and aspect ratio of the 17" tire, you may very well have a smaller circumference with a 17. Be aware that this will affect your speedometer and odometer accuracy. This can often be corrected by purchasing an "electronic tuner" than can adjust and compensate for the difference in diameter.
I know that we are talking truck tires here but I did just that with my van tires. I went from a 225/60/17 to a 225/55/17. This gave me a stiffer sidewall and almost an inch in overall tire diameter reduction. I made sure the tires were close to the same load rating (the tires exceed my axle ratings by a healthy margin) and were actually "H" speed rated also. It was not my intent to speed with my rig but to me it stands to reason that a tire rated for a higher top speed must have a more robust construction. I don't care about winter, I have dedicated winter tires also. I have noticed about a 200 rpm higher rev with my "summer tow tires" vs. my winter tires (which I had switched to 16" rims for a higher sidewall). The same thing would be evidenced if I had changed the gearing for my van (which happens to be 3.28's for an AWD sienna) I now have gearing around 3.38's with my smaller tires.
Interestingly, with the factory sized tires (my winter tires are very close to o.e. equipment) the speedo is appears to be out by 2-3 km/hr compared to my garmin gps. When my summer tires are installed the speedo and gps are in harmony