Sorry if this is a stupid question, but I've never set one of these up myself. My tire size says 26x1.75x2. The computer has entries under 26 inch wheels for 1.9, 2, 2.1, ect, but not 1.75. So which one of the numbers on my tire do I go by? the 1.75 or the 2?

Forget the canned size chart. Measure the distance in inches that the wheel travels in one revolution, using the valve as a marker. Convert the inches to mm by multiplying inches by 25.4 That is the number you input to your wireless computer during setup. It will be more accurate if you are on the bike, and someone helps push you, but that borders on overkill. Good luck.

Thank you very much. I figured out how to do the custom setting. The tires are marked to be inflated from 40-60 psi, so I inflated them to 50. I then sat on it as best I could here in the living room without risking major injury while marking out one revolution of the front tire starting with the valve stem down. That came to 78.5". I multiplied that by 25.4 and that came out to 1994 and that's what I set it at.

Back when I cared whether it was really accurate or not, I did my first calibration with a rollout and some chalk, then I rode about 20 miles with a hiking GPS velcro'd to the bars and assumed the GPS was going to be pretty accurate. At 20 miles it was about 0.2 miles short, so I adjusted the value up by 1%.

These days I don't worry about it all that much, I just use the same value of 2155 on my old bike regardless of what wheel I put on it. I did do a rollout on the new road bike, and that came to 2095 (the old bike generally has 32 to 35s on it, the new one, 23s)

WOW! Did that ever work. There's a trail close to me that has a one mile stretch marked off. I set the trip feature at the first post and it turned over to one mile within inches of the one mile post. I did about a dozen miles and then did it again on the way back. This time it was within about three or four feet. I'm in the National Bike Challenge this year and so I wanted it to be as accurate as possible, but I never dreamed it would be this close.