A few months back, I rode the Prudential London-Surrey Ride 100. That’s a 100-mile sportive, except that my Garmin recorded the route as 155.9km, or 96.871769 miles. Somewhere it seems I missed about 5 km/3 miles… or maybe I just cut all the corners!

Well, even though I was using GPS, I may tonight have found something that might account for a little bit of that variation – it seems my Garmin cycle computer was set to the wrong wheel circumference. Not wildly out but about 0.8%, which won’t help.

The Edge 810 software has options within a bike profile for both manual and automatic wheel size adjustment.  For my Bianchi C2C profile, it was set to automatic and had decided that my wheel circumference was 2088mm, probably when I originally paired my speed/cadence sensor as, according to the Garmin website:

“Wheel size is automatically calculated when a Garmin Speed/Cadence Bike Sensor (GSC 10) is paired to a GPS-enabled device”

With 700x23C tyres fitted, it should actually be closer to 2096mm (which seemed to be the default when I switched to a manual setting, or maybe that’s what I had originally entered before it was overridden by the software?) but I switched to 700x25C tyres about a year ago which will have a circumference of around 2105mm according to this table (and this one).

For completeness, I checked the profiles for my mountain bikes too – they both showed an automatic wheel size of 0mm (so presumably get their distance from GPS) but have now been changed to 2075 for 26×2.20″ and 2037 for 26×1.85″ (defaults seemed to be 2050).

As for the rest of the difference – well, tyre pressures are a factor – as is the weight of the rider. One school of thought says you should put some paint or water on your tyre, ride along and then measure the gaps between the dots. That assumes you ride in a straight line and that the other factors (weight, tyre pressure, etc.) remain constant between rides.

If the organisers said it’s a hundred miles, then I’ll go with that. Hopefully now I’ve amended the wheel circumference that will help a bit in future though.