Tire Calculations

There are a couple of calculators accessable here. The first one, allows you to input a tire size, such as a current tire you are running. There is a second input field, which allows you to input a secon tire size, such as one you are considering as a replacement, then, you can see how the tires differ.

A more full featured version appears here.

I used to have a Tire Plus Size Calculator link here, but the link is dead now. Plus sizing for the unaquainted, is the practice of increasing your wheel diameter and tire section width, without increasing your tire diameter. The wider tire, changes the shape of your contact patch, and also distributes the work done by the tread, over a larger area. Tires last longers and handling improves. If you change the width only, you have done a plus zero. Plus one and higher,  allows for a shorter sidewall, which makes sterring more responsive, and the tires grip the road better, because there is less sidewall flexure. Tire wear is improved also, but the ride gets a bit stiffer. If you have 15 inch wheels, and upgrade to 16 inch wheels, you are doing a plus 1, going to a 17, would be a plus 2 etc.

Rule of thumb for plus sizing is:

Plus 1 - Increase width by 10mm, wheel diameter by 1 inch, and reduce aspect ratio by 10

Plus 2 - Increase width by 20mm, wheel diameter by 2 inches, and reduce aspect ratio by 20, not exact, but close enough.

You can go here to read about the tire plus size calculator program you can download for a Palm, I believe.