Garmin: GPS vs Sensor

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BrynCP

Über Member
Location
Hull
I bought an Edge 1000 with a sensor for the wheel, this takes over the distance/speed calculations from GPS.

The sensor can be automatic or you can enter a wheel size. My tyres at 700x25c, using the Garmin table, it should be 2105.

However, if I let it auto calibrate, it auto calculates to 2085, which would be 700x20c. On a route that has always measured 18.6 miles via GPS, it comes out with this as 18.75 miles.

If I enter 2105 and turn off the auto calibrate, the same route measures 18.9 miles.

That appears to be quite a discrepancy, there are no steep hills on the route and there is 152m of elevation gain in total. I guess the only way to be 100% accurate is to measure the tyre its self!?
 
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T.M.H.N.E.T

Rainbows aren't just for world champions
Location
Northern Ireland
That's really nothing to worry about
 
That's only a 1.5% (approx.) total discrepancy from your highest to lowest figure. Looked at another way, it's +/- 0.75% on the middle (18.75) figure (again, approximately). That's pretty accurate. If I were you I'd accept the auto-calibrate figure and not worry about it.

That said, the wheel size figure (2105) should, in theory, be most accurate in normal circumstances, so you could perfectly justifiably choose to go with that one. GPS figures will always be inclined to err on the low side for moving objects on curved courses, and especially where the curves are sharp, since they're measuring a series of straight line segments, not the full curve. If you have it sampling at one second, the result will be close to your actual distance, but throw in a few positional wobbles as you lose/gain satellite coverage and +/- 1% (again, for a moving object; you on a bike) is not at all bad really.
 

bpsmith

Veteran
If you're questioning it, I would say you have to measure the tyre to be sure. I know I would.

Easy way is to mark a line on the road in chalk. Line the valve up with it, then wheel the bike along to rotate one full turn and mark again where the valve meet the bottom. Then obviously measure between both marks. Make sure tyre pressures are what you normally use first.

Let us know what yours measures @BrynCP.
 

raleighnut

Legendary Member
If you're questioning it, I would say you have to measure the tyre to be sure. I know I would.

Easy way is to mark a line on the road in chalk. Line the valve up with it, then wheel the bike along to rotate one full turn and mark again where the valve meet the bottom. Then obviously measure between both marks. Make sure tyre pressures are what you normally use first.

Let us know what yours measures @BrynCP.
If you want to be really accurate you should also be sat on the bike when you do the rolling bit, but bear in mind that after a couple of hundred miles the tyres will be worn so will need recalibrating. :whistle:
 
Well to be /really/ accurate then, having measured the tyre very accurately (not that easy, imho) you should then cycle precisely along a 'measured mile', if there's one near you, and then correct it based on the discrepancy you'll undoubtedly see in the reading.
 
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BrynCP

Über Member
Location
Hull
I think it's just my OCD kicking in and I know I shouldn't worry really, but I like things to calculate.

I will probably measure the tyre/wheel as bpsmith says though, out of interest to compare against the Garmin tyre size table, but stop worrying about the difference!
 
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