gps vs cycle computer

[mogse]

i've fitted a cycle computer to my road bike but i also have a garmin 305
i've done several training routes on it now. my cycle computer is reading much higher speeds and further distances than the garmin gps? i trust the gps more than the bike computer but would like the cycle computer to be more accurate.
my wheel size is is 700x 23 so ive set my computer to 2133 as per the instructions.
the wheel magnet is on the spoke about 4 inches down from the inside rim.

any ideas guys? or are cycle computers generaly inaccurate,
could you recommend one that is fairly accurate?

thanks

[Tmax]

. my cycle computer is reading much higher speeds and further distances than the garmin gps? i trust the gps more than the bike computer but would like the cycle computer to be more accurate.
my wheel size is is 700x 23 so ive set my computer to 2133 as per the instructions.

thanks[/QUOTE]
It sounds like the 2133 number you entered for the wheel circumference is too large. For every revolution of the wheel, your computer thinks you are traveling farther then you actually are, plus would indicate a higher speed.

You should do an accurate measurement of your wheel's circumference instead of using the one out of the manual.

[9202]

i've fitted a cycle computer to my road bike but i also have a garmin 305
i've done several training routes on it now. my cycle computer is reading much higher speeds and further distances than the garmin gps? i trust the gps more than the bike computer but would like the cycle computer to be more accurate.
my wheel size is is 700x 23 so ive set my computer to 2133 as per the instructions.
the wheel magnet is on the spoke about 4 inches down from the inside rim.

any ideas guys? or are cycle computers generaly inaccurate,
could you recommend one that is fairly accurate?

thanks
If I remember correctly, a 700x23 wheel supposed to use a 2096 circumference, I have always set my Garmin Edge 305 to auto detect the wheel size and it seems to be accurate.

[Hideandwatch]

If I remember correctly, a 700x23 wheel supposed to use a 2096 circumference, I have always set my Garmin Edge 305 to auto detect the wheel size and it seems to be accurate.
Yes definately 2096. Anyway that is what it tells me to set it to in my Cateye Astrales 8 manual.

[mitosis]

i've fitted a cycle computer to my road bike but i also have a garmin 305
i've done several training routes on it now. my cycle computer is reading much higher speeds and further distances than the garmin gps? i trust the gps more than the bike computer but would like the cycle computer to be more accurate.
my wheel size is is 700x 23 so ive set my computer to 2133 as per the instructions.
the wheel magnet is on the spoke about 4 inches down from the inside rim.

any ideas guys? or are cycle computers generaly inaccurate,
could you recommend one that is fairly accurate?

thanks

Do a roll out. Mark a point on the ground, roll the bike two or three wheel revolutions (while sitting on it with riding tyre pressure) and mark the ground again. Measure with a tape, divide by two or three (depending on whether you rolled it 2 or 3 times) and there is your wheel circumference.

[fleshbroiler]

I've noticed the same thing but chances are the GPS distance and speed data are low, if you are using "SmartRecording." SmartRecording mode is required for uploads to the MotionBased website. With "SmartRecording" data is recorded at varying intervals up to 10 seconds. At 20 mph you're moving ~29 feet per second. Your distance travelled is calculated from the distance offset between trackpoints, not where your wheels have rolled. You can view this by uploading your gps data to motionbased and zooming in to an intersection. The plotted route does not follow the road network. If you have the means to view data recorded at one trackpoint per second, you will see the difference. Changing your data recording rate to 1/second should improve the accuracy of your route distances and average speeds.

-fb

[pistole]

2096

[mitosis]

It varies with brand of tyre, tyre pressure, your weight, just to name a few. If you want it to be accurate measure it. It'll take as long as finding the manual and looking it up. :)

[Powerful Pete]

Yup. Roll it out and measure, if you want half decent accuracy.

[mogse]

Do a roll out. Mark a point on the ground, roll the bike two or three wheel revolutions (while sitting on it with riding tyre pressure) and mark the ground again. Measure with a tape, divide by two or three (depending on whether you rolled it 2 or 3 times) and there is your wheel circumference.thanks for the replies

i did the rollout thing, measured 2 revolutions, got 420cm (4200mm) divide by to works out roughly 2100 some of you did mention 2096 . i'll give that try see how its matches to my gps data. last time i go off a manual again

ive been uploading to motionbase what a great website much better than my garmin training software
thanks again