Last week I purchased a new wireless Cat Eye Cyclometer, model MC100W.

A tech at the bike shop calibrated it for me based on my bicycle wheel size. He had a tricky time with it, and complained that Cat Eye had changed their process for programming their cyclometers after YEARS of doing it ONE WAY for most models. I said "better you than me; I'm not too swift programming computers."

Well, having just started commuting with it, I noticed on the commute today that the total miles once I reached work was about 1 mile LESS than what my previous Cat Eye used to read on a different bike (calibrated by a different tech.)

So either the new Cat Eye is wrong, or the old Cat Eye was wrong. If the new Cat Eye is wrong, and as a result it's giving me total mileage that is LESS than what it should be, that would also mean that during my ride the speedometer is giving me a lower number than what I'm actually doing, correct? So if it says I'm doing 15mph, I might be really doing 17mph.

Do I have that right?

Well, I don't know what specific model you have...but I was able to figure out how to calibrate mine pretty easily from the manual. I have three different models..all on bikes with different tire/wheel sizes and I get the same basic readings for my commute on all three bikes. Maybe the wireless ones are more complex...I don't know, but you can download the manual for each model on their website.

Check:
https://sheldonbrown.com/cyclecomputer-calibration.html

I use roll out method and both my bikes match to .01 mi for the same 9mi route.

Al

All the owners manuals can be downloaded from the Cateye website. This would let you check the settings for your wheel size.
Many things including some bike headlights will intefere with wireless bike computers, you may just have some interference.

It is likely set for the wrong tire size. Some new bikes come with a 700x26 tire that isn't even listed in the owners manual.

Speaking of interference, mine was a pain to set up but not due to the unit. Something in the area of my stereo and TV interferes with the unit so when I was setting it up, it couldn't communcate between the sensor and unit. When I tried later across the room, it worked great. I can spin the wheel near the TV and nothing registers at all.

YES!!! OK, this little dumb arse (me) figured it out.
The tech at the bike shop had my cyclometer set at 2070 instead of 2170.
Which translates to this: The Cyclometer thought my tires were 700x18 instead of 700x30.

WHEW!!! So maybe I'm not as much of a lard a$$ as I thought?

I'm a big fan of using the google maps pedometer to plan routes and check distances from home. Get zoomed in on your area and then switch it to the hybrid view to see sat photos overlayed with street maps.

https://www.gmap-pedometer.com/

Pretty accurate! Thanks for making sure my CatEye is correct (Astrale 8)

UPDATE: Commute home made me feel much better about myself, as the cyclometer was now producing accurate readings after my calibration tweak today.
Amazing how self-esteem can be tied into something like this.

Tonight on the commute home, in the dark, in the rain, my average over 16 miles was 15mph, and my maximum was 25mph... and that's on a singlespeed bicycle. The moments when I hit 25 in the rain in the dark felt dangerous to me so I backed off.

I have a friend who has a computer that is 10% optimistic and refuses to change it. That way she rides 10% longer than I do !

[QUOTE=igloomaster]YES!!! OK, this little dumb arse (me) figured it out.
The tech at the bike shop had my cyclometer set at 2070 instead of 2170.
Which translates to this: The Cyclometer thought my tires were 700x18 instead of 700x30.
QUOTE]
Thats less than 5% off. I dont know why everybody gets so concerned about the accuracy of their computers. I like to know how todays speed compared with yesterdays and my PB. If you want to brag about your speed and distance set your roll out to 2370.

[QUOTE=AndrewP]Sorry, hope I didn't offend you, certainly didn't mean to, and I'm certainly not bragging. I did call myself a dumb ass. My average went from 12.5mph when the cyclometer was off to 15mph after the cyclometer was properly adjusted. That pleased me. Maybe it's the little things, that's all.

See. And you mom always said TV ws bad for you . Seriously TVs and stereos have a lot of RF (radio frequency) circuits generating radio waves. Even with FCC shielding there is leakage to interfere with nearby wireless bike computers. Same reason you must not use a cell phone in a hospital. Also the power supply in a HID bike light that jumps battery voltage to light voltage can generate RF. If you really must have the bike near the TV try a wired computer. And you might need to put shielding on the wire.