Tire Size and Circumference As the rear tire is a major part of the total gearing of a motorcycle it is important to use the correct size of the rear tire in the GC. The tire size consists of 3 fields, the tire Width in millimeter, the tire Aspect (= height in % of tire width)  and the Rim diameter in inches. Not very consistent but that's the way tire manufacturers do it. An example is: 190/50/17. This means 190 mm wide, 50% of 190 mm high and a 17" rim. Using this data the GC calculates the tire circumference which is used to calculate the speed of the bike. Theoretically, a Bridgestone tire 190/50/17 should have exact the same sizes as a Michelin 190/50/17 but in reality all tires are different. Hell, even 2 Bridgestone's aren't the same. Because this influences the accuracy of the GC, as of version 5.1 I have added a field called 'Tire Circumference' which you can use to adjust the calculated to your actual tire size ! GC uses the default tire size from its database to calculate the tire circumference and shows it besides the tire sizes. Now if you know that you tire circumference is different you can enter it in the field called 'Tire Circumference'. GC will then recalculate the sheet using your entered actual circumference. What it also does, is adjusting the tire aspect field to match the new tire circumference. Normally this field is like '50' or '55' in percent. After manually changing the calculated circumference, GC recalculates the new corresponding tire aspect where Width and Rim will remain the same. So the new Aspect could become '50.4' or so. How to measure your actual Tire circumference: This is all great but now you need to find out your actual rear tire circumference. You could wrap a tape measure or a string around your rear tire but that is quite complicated. You could also do the following: Mark the rear tire exactly in the middle of the tire with a tiny drop of paint or tipp-ex, white-out (correction fluid). Now put the bike on its wheels and move it forward IN A STRAIGHT LINE at least 2 tire rotations Put the bike on the jiffy and measure the exact distance between the first and the 3rd dots on your floor Divide this by 2 and you have your actual tire circumference ! This is the value to manually enter in the GC. Make sure to use the right units 'Inch' or 'mm' depending on the selected dropdown value 'Imperial or Metric.