nomadmax 

This year I upgraded from a watch strapped around some pipe insulation on my handlebars to a Cateye bike computer. Recently, I've ridden with folks who have GPS based cycle computers and in comparing the data I'm always 2 or 3 tenths higher in mileage. I intend to change the input number and believe if the number for 700x25 is 2105 and 2096 for 700x23 then I should bump the number to 2110 or so to get it closer to accurate. Does increasing the number decrease mph and miles or is it the other way?

Weakbikr

I had the same problem, my numbers never match with gps, close but not the same. If my thinking correctly, bigger numbers slow it down, smaller numbers speed it up. Now my suggestion is air the tires up to the pressure you ride. Make a mark on the side wall on the rear tire. Make a mark on the floor. Have a friend help you. Line the mark on the tire to the mark on the floor. Sit on the bike and roll forward until the mark on the tire makes one revolution. Then make a mark on the floor. Measure the two points and input the measurement in your computer. Also save that measurement. It's still not going to be the same as gps but you know your measurement is now correct for your setup. There are videos that will show you how if I wasn't clear on the process. Hope that helps.

Weakbikr

Sorry I went on, short answer is yes increasing the number will decrease mileage.

waters60

Roll it out to get your exact number, if Cateye has a place ti put that number.

deacon mark

Cateye does have place you just input the wheel roll out in mm. On my Habanero and Wilier they
are both 2093. I use that number and rarely are total amount from my Garmin 910 much different. The difference is less than 1/2 of 1 percent and I have done 50 mile rides and be like 50.25 and 50.35.

The measure the same thing but differently, will not be exact but i bet mine is down to less than 10 feet in a mile.

nomadmax 

Thanks all!

BCDrums

I use a Cateye and I compare its recorded distance to the distance I get on Ride with GPS, and at first they did not match—the Cateye was higher. So I began to decrease the circumference number in the Cateye by 5mm at a time until they were close, but still they never match. Finally I decided there was a little bit of deviance in both.

RwGPS shows a distance in tenths, whereas the Cateye gives it in hundredths. But if I switch to the RwGPS Edit Ride page, and Show Metrics, it gives a number in ten-thousandths, e.g. XX.xxxx. I find it hard to believe that is accurate; a ten-thousandth of a mile is 5 feet.

nomadmax 

Wouldn't decreasing the circumference number increase the error even more if the error was optimistic ?

BCDrums

If by optimistic, you mean higher, then no. Decreasing the circumference will give a shorter total distance. Distance is calculated by revolutions x circumference, so the smaller the circumference, the shorter the distance.

I like that use of optimistic, though.

nomadmax 

Got it. so in the case of the OP, an optimistic (too high) reading of mileage @2105, 2100 would be moving toward correction, correct?

There are a few things I'm good at, numbers, shoes on or off, is not one of them

BCDrums

Yes, that is correct.

I share your pain!

jadocs

I used to measure out my tire and input manually on my old computer. I marked the tire with chalk and actually sat on the bike to roll it out. I figured the weight of my body on the bike would compress the tire and I wanted to get the most accurate measurement I could.

nomadmax 

Here's the saddest part of all. In a year when the battery goes dead, I'll do a search, find my own post and re-learn what I know now

BCDrums

I have done that too. It's probably as accurate a number as you'll get for the computer. But I decided I wanted to get RwGPS and the Cateye to match, so I fiddled the Cateye number until it (mostly) did.

Abe_Froman

Somebody earlier had this incorrect. Larger input tire circumference will result in the cateye showing higher mph and distance covered, NOT the other way around.

If cateye is overshooting the mph and distance, you want to lower the input tire circumference. All it is doing is taking wheel revolutions x wheel circumference to get your distance traveled. If you reduce tire size, you reduce distance.