So I have started to ride with a group now (which is way more fun than alone), and at the end of our long rides we all compare computer information, average speed, etc.

What I have noticed is that most of the people I ride with all have near the same results in distance and speed and all. My computer seems to be off... and I mean by like a half mile.

So I started to think the programing was off, tire size in the computer wrong.

So I looked on my bike to double check tire size and came up with some interesting discrepancies.

My rims say 700x25c
however my tires say 700x23c

I am not an expert on this, but my computer is programed based off my tires the 23c, would that make a difference that much? should I keep the 23c setting or change it to a 25c?

Odd how when everyone rides the same distance, all of their averages are the same. Grab your setup manual and reread the Calibration section.

It's the tire that makes the difference to the outside diameter of your wheel, so leave the 23C.

For the ultimate in accuracy though, try measureing the OD of your wheel. Line up the stem on the ground, mark it, roll it till the stem is in the same place and measure the distance. Usually computers want the distance in cm or mm.

There will be a slight but inconsequential differnce between the 23c and 25c sizes. A decrease in width will also slightly change the profile, thus giving a different length of roll per wheel revolution.

I marked 100 feet to the inch with a contactor's tape measure, then made some faint paint marks on the road in front of my house. It's straight, flat and fairly level, which you need for good calibration.

I put white tape on my front rim for visibility. I start at first mark with rim tape aligned to paint mark, ride slowly to second mark, and stop dead center on end mark. While rolling I count revolutions. At the end mark I determine fractional circumference by counting spokes from tape mark, ie, if I'm 8 spokes past the tape mark to the 100 ft paint mark, then I know the last revolution was only 8/32 = 0.25 revolution. So I completed X plus 0.25 revolutions.

Then conversions. Convert x.y revolutions in 1,200 inches to inchs/rev, which is circumference in inches. Convert to mm or cm as required. Enter conversion factor into computer.

Make sure tires are inflated to pressure you intend to ride before rollout procedure.

Also, you'll get the best results by getting an assistant to push you slowly over 100' calibration course - let them provide the push, you focus on steering a perfectly straight line. This is hard to do at very slow speed by yourself, while you're staring at your front rim and counting revolutions, plus aiming for the finish line paint mark. Probably would be easier if you have a painted line to follow, with calibration marks added.

If done properly, this method is accurate to 1/1000. On several occasions I compared my (cateye) reading to garmin gps, had agreement of ~0.05 mile over 40 mile course.

Or, 0.05/40 = 0.1%. Good enough.

When measuring your wheel outside-diameter using the above method (noting where the tire stem comes to ground twice), do it while sitting on your bike with your full weight on it, in normal riding position. This will compress your tires slightly and give the exact same result during calibration, as you get when riding the bike.

As seeker333 said, you might have to get someone to help when doing this, holding the bike upright and pushing you while he walks alongside and marks each exact position where the stem "touched" the road.

If you simply walk beside the (empty) bike and mark where the stem hits, you will be off by just a smidgen. Maybe not enough difference to matter. But hey, if you want to do it "right"....

Cateye computers come with a very complete list of tire sizes, and what calibration number to use for each. I had one on a road bike, and put in the exact number they recommended for the 700Cx25 tires on it. I then rode around the bike path on the local lake (Miramar), which has mileposts every 1/4 mile. I know, there's no guarantee that those posts are exactly correct. But I noticed that the first post (1/4 mile) and the 3-1/4 mile post, were EXACTLY three miles apart according to that Cateye computer, right down to the foot.

I figure that either the Cateye computer and the mile posts were off by the exact same amount and in the same direction... or else they were both correct. Choice B seems more likely to me. So I've been using those two mileposts to calibrate every computer I've had, ever since.