In July of 2023 I got a new Trek AL 5 and go the usual accessories with it. Bottle cages, seat and cycle computer. It is a Bontrager RIDEtime and was set up by the Trek bike shop. On my 1997 Trek 1220 I had set it up myself using the roll out wheel size method. This time I figured with a bike mechanic doing the set up he would get it as accurate as possible. My mistake for making that assumption. It is a wireless unit and I didn't want to try and fail setting it up myself. Fast forward one year. I have. been questioning my speed on the new bike compared to the older one. Am I just slowing down a lot faster than I thought I would at 76 years old? Or would the rollout method of wheel sizing be better than the wheel chart? The wheel chart says for a 700 x 32 wheel the setting is 2190. But the roll out method comes up with 2140. I have changed it but haven't ridden yet. Will the 50 mm difference matter on speed and how much difference will there be on a ride of twenty miles. I am not a math whiz so I don't know how much difference it will make. I tried doing some calculations and couldn't really get anything that sounds logical as a result. Anyone have a quick calculation that might show me what I should expect with the change? Thanks.

Charts can be wrong. Certainly they don't take all the variables into account such as the actual height of the tire on your rim or how much pressure you put in your tires which also affects how far you travel every revolution.

So doing a rollout will very likely always give you the best results. You can calculate the circumference of the tire also by assuming the tires height is also the same as it's width. So your 32 wide tire comes to: 3.14 x (622 + (2x32)) = 2154 mm. Which is 14 mm different. And will only be 1% error.

However the charts figure is still only a tad more than 2% error. Way better than your cars odometer is required to be.

i input 2128 in my bontrager computer and with my conti 5000 700x32 tires I am spot on compared to using the strava app. ie with in a few tenths on a 100 mile ride. can't really get any closer than that.

You're talking about a difference of a little over 2% between the 2190 and 2140 rollout terms, or half a mile over 20 miles. Does that matter to you? Speedwise, probably not; distance, it could make it hard to follow a cue sheet, especially in town where there are so many possible turns. FWIW, I usually start with the next smaller size wheel, so if I'm riding 700Cx32 tires, my weight compresses the tires while riding so the x28 tire factor gives me a better starting place. Rollout measurements are a little better for me. If you care, you may want to do a fine calibration.

To truly calibrate your computer, you need a good standard to compare it against. I like to use a ridge near my house to do coast-down testing (so I don't have to worry about swerving while pedaling); it's about 2 miles long with surveyed mile markers that seem to be trustworthy (unlike some states' mile markers). If you've got a decent GPS, or something like the Strava or Ride with GPS app, you could use that as your standard. Find a course with unobstructed sky view of a mile or two, or ten. (Unobstructed view will give you good GPS reception, without dropouts or interference from tall buildings.) Reset your computer, start the GPS or app, and ride for a while. Stop and record both distances.

Calculate the correction factor: "measured" (by the computer) distance / "real" (app or GPS) distance. Multiply that correction factor by the "wheel diameter" measurement in your computer, and you have the better wheel diameter. Note that I've had computers that matched mile markers or GPS for longer rides to within 0.5%, but that usually changes with tire pressure and/or tire wear. If it's better than 1%, I enjoy being smug. At 1%, I don't bother trying to improve.

(2190 - 2140) / 2140 ≈ 2.3%

This is one reason I prefer a GPS bike computer. It'll do the calculation for you and calibrate. Both speed and distance will be accurate.

Yeah, but give a man two gps's or a map and a gps and he'll never know what distance is accurate.

When I do the rollout I sit on the bike to compensate. I'm not sure if I can find a place to do the comparison of my GPS unit and the cycle computer. But I think I need to try something. I went for a 25 mile ride today, one of my usual routes, but I threw in a few extra sections. When I was home I went onto mapmyride.com and plotted out the route. I got 25.31 miles. I think that is a significant difference. So, now my question is this. Since I changed the wheel size settings, is that causing the difference? Will a smaller wheel size make the distance traveled more or less than a larger wheel size? Or is it the reverse? My mind hurts when I think about this.

IMHO, it's time to forget about a 1.01% difference. I like to shoot for 0.5%, but I enjoy the tinkering. Since you don't like dealing with the math, don't! I've watched people argue about whose computer or GPS is right when they differ. Unless you rig something to leave a paint trail on the road or path and then hire a surveying crew, you'll never. Just enjoy the ride.

It'll still be a lot closer than the 1% we are speaking of here, at least 2 GPS's will be. Maps themselves are just difficult to measure, even if the printed scale is dead accurate, you don't have a device that is so precise that it'll read properly.

A smaller wheel size will read faster and farther because it is rotating more often to cover the same distance as a larger wheel. HTH