So I was wondering if any experienced persons had some ideas. Just got my new road bike with 700x23 tires. LBS sold me a Topeak odometer at cost to get rid of stuff for Trek Bikes(lost trek dealership). Anyway my buddy and I went for a ride today and it was my first trip out. We stopped for water at a point and I was mentioning out mileage. Mine said 9.63 miles and his was almost 12.5. I believe he has a cat eye odometer. His gps on his map my ride app was showing the same mileage as his odometer. I checked the settings and my computer was set to 2096C tires which is from what I can tell the correct setting. Any ideas? Thanks in advance

Most of these cycling computers I have seen ask you to enter a circumference for the tire and sometimes give you a table to look up the circumference for standard tires sizes. Don't trust the table values. The better way to make sure you are entering the right value is to actually measure the tire/wheel combo you have. To do this, find a flat section of pavement (outside) or floor (inside) and position the front wheel with the valve stem at the very bottom of the wheel. Mark this position on the ground or floor. Roll the bike forward in a straight line until you get the valve stem again at the very bottom of the wheel. Mark this position and measure the distance between the two marks. Convert this to millimeters and then enter the value into your cycle computer.

I'll give that a shot. Thanks

So I went by the bike shop and after the guy thought about it for a minute he lowered the number in the computer to 2084(I think that was it) which was the best size down. Does that sound right to you guys? To recap my computer was 3 miles short of what my buddy had on his comp and gps.

I did the wheel test this morning and if I did it correctly I got 83.25 inches for the roll test on a 700x23 bontrager tire and wheel. When converted to mm the number I got was 2114. Which one sounds more correct to you experienced guys. Sorry for the beginner questions

2114 would be the better number to input since you measured it yourself. Remember, tires of the same size can vary, so if you want to be as accurate as possible that's why many measure it themselves.

Most bicycle computers use the mm measurement and you can input whatever number your measurement is. The only exception I encountered was with the Sigma computer, pre 2012 models, which required you to divide and multiply to get the input number if the chart number recommendation provided didn't match closely enough to be considered accurate.

In the OP, the reported distance is about 23% lower than that of his friend. That's a big discrepancy and the suggested changes in the circumference between 2096mm, 2114mm, or 2084mm are very small in comparison (less than a 2% variation). So there must be another reason. Either this cyclometer uses something other than the usual circumference in mm as the distance setting, it's defective, or possibly the magnet is placed a bit too far from the sensor and some of the distance was not recorded.

I'd first check it again over a known distance and verify during the ride that the speed is showing up properly the whole time instead of sometimes dropping to '0' for a little while.

If your friend's Cateye was calibrated with GPS, your spoke-magnet Topeak unit isn't working.

My Topeak overeads my Garmin GPS by 0.01 mi/ 1 mi, or 0.1 mi/ 10 mi, or 0.3 mi / 30 mi. Trying to adjust it to closer tolerances by reducing the entered tire circumference, doesn't work. It undereads. So, I'm happy with 1% error.

Try resetting your tire circumference setting empirically by 25%. Is that working? If so, Fine tune it.

If your Topeak's magnet isn't strong enough, even at 2 mm from your sensor you can buy a cheap neodymium/iron/boron magnet to self-attach to your Topeak magnet.

If that doesn't work, it's either your head-unit /bar base connection (use fine sandpaper to scrub off oxidized corrrosion) or your head unit computer is defective.

23% is too big to be a calibration issue, it's probably the magnet too far from the sensor. You'll probably need to make sure the magnet is proud of the spokes, and then rotate the sensor around on the fork to get it as close as possible to the magnet without touching. Also make sure the magnet is correctly located on the spoke, so the center of the magnet passes at (or very close) to the centerline of the sensor. (The sensor centerline is usually marked with a line on the plastic housing.)

Also, place the magnet and sensor closer to the hub rather than the rim if it's not already. If they're too close to the rim the magnet can pass the sensor too quickly to register.

Magnet should be no more than a millimeter or so from the sensor, and both as close to the hub as you can make them. Putting it too far out will make the magnet fly past the sensor so fast that when going at higher speeds the sensor might start missing pulses.

Lotus907efi tells the truth, it's very important to get the correct size of your tire.
Another thing may cause the mistake is, many simple stopwatches don't have speed-sensor, that makes the calculation not precise.