I have a bike computer, an Axiom Wireless, that I put on my mountain bike. I had it on my comfort bike (26" wheels) and it didn't seem to have a problem but now that it's on my new MB (29" wheels) I wonder whether
the speed and distance, mainly distance, it is showing is accurate. It seems high, I noticed it right away because I ride in the same places I did my other bike and the numbers aren't the same. When I put the computer on the new bike I measured the wheel by rolling it and marking the distance of one revolution. I put the number (90" x 25.4) of 2286 into the computer for tire size but I think some thing is wrong. For instance I road a trail today that is 3.8 miles long, the computer read about 5.3 miles. Now the number on the trail map, 3.8, maybe off but I don't think by 1.5 miles. Any ideas?

Two things can affect the readings.

One, if you put the magnet out towards the rim, at high speeds the magnet can be passing the sensor so fast that it doesn't register. It's better to put it closer to the hub.

Two, if you mount the magnet incorrectly, sometimes (probably at lower speeds) the sensor might register each revolution twice, because it will pick up both the north and the south pole of the magnet as a pulse. Most magnets I have experience with wouldn't really allow this but some might.

3.8 miles = 6.1 kilometers. Still even if it was displaying Ks, that's pretty far off from 5.3.

Silly me. JerrySTL you nailed it, at least part of it. I had changed the mph to kph accidentally and didn't notice it. When I changed it just now the total distance number changed from 59.40 to 36.91, that sounds more like it. The trail measurement may be off some. 5.3 k is about 3.5 m.

ItsJustMe, you know I had the sensor and magnet near the hub and because I had changed the computer display from mph to kph, I thought I was getting a false measurement so I moved them out toward the rim where they are now. I will see how it goes and move it if I have to.

I think you have input the tire size incorrectly. You should be inputting the actual tire size ( 29" x thickness). I've never seen a computer that actually asks you to measure the circumference of the tire.

Faster speed and longer distance would be a symptom of the computer thinking you have a larger tire size than you do.

The instructions for the computer said you can measure the circumference of the tire and enter it as a four digit number. The four digit number is the circumference of the tire in millimeters. The instructions had a list of tire sizes but 29" wasn't on it so I measured it myself.

I found a measured mile from my local running marathon. I just rode back and forth and adjusted accordingly.