Garmin/Cateye tyre size, distance discrepancy

[Paul139]

The wife and I went out for a ride recently and got very different distance results.
My wife has a Cateye wireless computer I have a Garmin Edge 510 and we also used Strava on a mobile phone. Now I realise there is always going to be some discrepancy between different recording devices but I wondered if the tyre size entered in each device is correct.
We are both riding on 700x25c tyres but the Cateye paperwork says set tyre size to 2105mm and I have always had my Garmin set to auto which sets a size of 2114mm. I realise Strava does not have this calibration function (as far as I am aware)
On our ride My wifes Cateye recorded a distance of 13.8 miles, her phone running Strava recorded 17.9 miles and my Garmin recorded 14.8 miles.
What size do you set your device to?

 

[Rickshaw Phil]

For accuracy I'd suggest measuring the actual rolling circumference of the wheels and entering that. The figures given in instruction leaflets are only a guide and aren't often accurate.

Having said that, you are looking at over a 20% margin of error between the Cateye device and Strava - something is really not right there.

 

[Banjo]

Measure a ride on map my ride or whatever.

Ride it and make a note of your distance from the cycle computer.

Work out the percentage difference between the real distance and recorded distance.

Apply this percentage to the inputted wheel circumference .

Try the ride again , this time it should be close to accurate.

 

[Paul139]

@Rickshaw Phil, I measured the wheel circumference which came to 2117mm which pretty much matches up to the auto measure figure of 2114mm my Garmin gives for my wheels so that seems better.
@Banjo, I will give your suggestion a go next time we go out, Thanks both of you.