Something interesting happened this morning- i rode section of rail trail that i've ridden hundreds of times in the past. i had a new cyclometer installed on my commuter bike yesterday-one that included cadence so i could watch my spinning as i've had some knee trouble lately. the new cyclometer clocked this section of trail at 8.5 miles. The cyclometers on my other 2 bikes have consistently clocked the mileage at 9 miles.
What gives?

have you correctly set the wheel size or was the old wheel size incorrect?

the bike shop installed it- another interesting fact that i didn't mention. when i was playing around with the new cyclometer i noticed there was previous riding information in the computer- you have me thinking- i'm wondering if this was a return and the lbs didn't bother to set the wheel information correctly for my bike?

You can probably find a users manual online. It shouldn't be that hard to set up. I'm surprised the LBS didn't give you a printed manual. Mine came with a printed manual.

They will only read as accurately as the calibration you use. Don't depent on (trust) the bike shop's value. What do they care? Measure and set it correctly and using the same method on all your bikes.

And I don't think any calibration is ever too precise. I can roll out my front tire over and over and over and never get the same perimeter down to the mm (never mind subsequent pressure changes etc...), although whatever rollouts I do get, they definitely aren't the same as the one listed in the table in the computer's manual...

Last time I checked, which was just yesterday by chance, my 700 X 23 tire was within 3 mm of the computers published circumference value (2133 vs. 2130) That 3 mm was well within my measuring error, since I used a wide piece of chalk and eyeballed it. So set your circumference value based upon the computers suggested value for your size wheel AND TIRE. But its not that hard to measure your bikes wheel/tire circumference: just put the valve stem at the bottom, make a mark on the tire and ground with chalk, roll the tire along the flat ground one revolution, make another mark, then measure the distance between the chalk marks. As I stated earlier, taking the 2 minutes to do this just verified that the manual recommendation was accurate.

On one bike I once had about a 2% difference, presumably due to the tread pattern being different than typical tires of that size. Being off 2% was no big deal, but neither was taking the 2 minutes to measure. Realize that your computer calculates distance and speed my measuring revolutions of the tire, and with a known circumference, revolutions multiplied by circumference equals distance, and revolutions per time unit multiplied by circumference equals speed.

A nearby state road has a measured mile for car odometers.
my computer tire values have allways come close enough for me.

And Kudos to Hot Potato as I allmost bought a box of sidewalk chalk yesterday.
Had this inkling it was relevant to something.

Even down to 10mm over a few revolutions on a rollout would be an error of < 0.1%, which would be more than acceptable. Compare that to the 6% difference in the two cyclocomputers the OP has.

I calibrate mine from the mile markers on our local rail trail. My cyclocomputer is extremely precise but (naturally) has no inherent built-in accuracy. The milemarkers are set by a GPS device, so they're very accurate over long distances - +/- 30 feet over as many miles as you care to measure. Doing it that way, I can get mine calibrated very well with very good accuracy and precision.

I also use the "measured course" approach. There's a loop that I cycle most days (when I'm not actually commuting) that is part of the course of a local 5K race. The "official" map of the 5k course shows the precise length of the loop. I usually do 3 or 4 laps of the loop and then compare the distance on the cyclocomputer with the known length of the loop (times the number of laps, of course).

This method gives good repeatable results, and as far as I can determine seems to have less measurement error than the "chalk on the garage floor" method (can you really measure millimeters accurately that way?).

Maybe you set the other two wrong? I decided I wanted to lengthen my commuter ride, but I didnt really have the time, so I just recalibrated my computers. smile

That gives me a good idea. If I ever start a LBS, I'll put computers on all the bikes. I'll calibrate the computer to give faster results progressively as the models get more expensive.

The half inch diameter pink childrens sidewalk chalk is my favorite. White gets confused with other stray marks on the garage or driveway, but its hard to miss the pink hash-marks about 84 inches apart, even in low light. And with two girls, there just always seems to be some on hand.

...what's the difference in precision between the two computers?

Think about it. Your tires are measured by the inside diameter. 700, or 27in, or 26in refers to the inside diameter of the tire (even that is not exact). Your cycle computer reading is based on the outside circumference.

I now own two tires, both 27in X 1 1/8 inch but different brands. The outside diameter of one is ½ inch larger than the other one. It’s so much bigger that I can’t use it with my fenders. ½ inch diameter means about 1½ iinches more in circumference (if I remember my geometry right). That has GOT to change you millage readings.

That's why the figures on the cycle computer instructions are only approximate.