Speed calibration affects odometer? EP800 / EM800

[RustyIron]

Some would say I'm too obsessed with data accuracy. But it gives my mind something to do, and it makes me happy. I run Strava on my iPhone, and I like the mileage reported by Strava to match the mileage on my cycling computer. On old-school cycling computers, it's merely a matter of entering the correct data for the tire circumference. Easy peasy.

But now I have this newfangled Em800 ebike computer. Supposedly, the assist feature shuts off at 20 mph. If the speedometer isn't reading 20 mph at the cutoff, there is an adjustment to change the reported speed. Although it's hard to measure, I think my speedometer is reading about 2.5% slow, but that's not what I really care about. What I care about is having the odometer on the bike match the distance reported by Strava. Interestingly, the mileage shown on the EM800 computer is about 2.5% lower than that reported by Strava.

So my question is, if I bump up the speedometer by 2.5%, will that added factor be reflected on my odometer? Will my reported distance be 2.5% greater?

 

[Zimmerframe]

If I remember correctly you're on an EP8. which I think generally seem to be set to under read slightly - but lots depends on tyre choice/pressure.

In theory if you change the wheel size by 2.5% then yes, that should relate to it showing the correct speed which will equally affect the odometer.

The only point of reference it has is the spinning magnet and the size it thinks the wheel is (which can also change with weight/heat bla bla ..)

 

[Tonybro]

Conversely, my E8000 motor over-estimates distance and speed. I have set it back by 2.5% and it is closer to my GPS distances. Probably needs to be 2.7% to be accurate...

 

[RustyIron]

I have set it back by 2.5% and it is closer to my GPS distances.

Cool. That's what I wanted to hear. It seemed logical, but not certain. I've always found that GPS and wheel-rotation distances are not exact, but if they're close, it doesn't stress me out.

 

[Husky430]

Also take into account that your speedo reads what the bike has travelled on the ground versus GPS looking at everything as a flat map not taking in the fact that you have gone up and down hills, it's the old "hypotenuse of the triangle" syndrome.
My way of understanding is that your speedo will always read more accurately as its wheels on the ground vs a beep from your phone being measure by a satellite 100s kms in space.

 

[RustyIron]

Well... yeah... no... sort of. The distance traveled can be measured as short as the distance between the starting point and ending point, and as far as infinity. The math is easy to prove. All I care about is figuring out how to make my two measuring systems come out equal.

Further reading: Benoit Mandelbrot, Fractal, Coastline Paradox.

 

[Gyre]

Also take into account that your speedo reads what the bike has travelled on the ground versus GPS looking at everything as a flat map not taking in the fact that you have gone up and down hills, it's the old "hypotenuse of the triangle" syndrome.

If your GPS has enough satellites in its navigational solution, your coordinates will be three-dimensional, not merely lat/long projected onto a spheroid. A very lazy algorithm *could* treat things as a flat map, but there really isn't any reason to do that.

My way of understanding is that your speedo will always read more accurately as its wheels on the ground vs a beep from your phone being measure by a satellite 100s kms in space.

That's not how GPS works, and I'll go out on a limb and say that the distance of the satellites isn't really a limiting factor.

Barring poorly implemented algorithms, GPS should have much better straight line odometer accuracy because the errors tend to average themselves out where wheel circumference errors accumulate with distance. Wheel circumference is better in twisty trails as long as you're calibrated properly because there GPS errors are at risk of accumulating quickly.