I did some reading/searching of tire size for computer input. I use a Sigma Sport 1606L on my bike and have always used the chart in the manual. For my tires, Continental Ultra Race, 700C x 25 I used 2146. Yesterday I did my commute from work to home on Ultra Sport 700C x 28 and I changed the wheel size setting to 2149 as the manual says. When I got home I saw that my distance, which was 12.21 miles on 25mm tires was about 1/4 mi. short. So obviously my 28mm tires are not 3 mm bigger in circumference. I attempted to roll out the wheel and measure but I need a better methodology. However, what would be the best way to convert the difference in measurement? The reason I particularly care is because I had a heck of a tail wind yesterday and was very close to beating my commute record.