Possible cause for Google Maps / bicycle odometer divergence (tire size setting correct)

Question

I have plot my exact route from home to my office using Google Maps. By exact, I mean that I took nearly every turn and change from auto-calculated route to match my exact route into consideration.

Google Maps claims that my route is 10.4 km. Everyday I'm riding it, my bicycle's odometer claims that I have actually traveled 11.11 km.

I did exactly the same with my return route (a bit different) and after including every actual turn, change etc. it turned out that route is 9.5 km according to Google Maps and 10.43 km according to my odometer.

The difference is 0.7 / 11.1 = 6.3% on the outward trip and 0.9 / 10.4 = 8.6% on the return trip¹.

My first and of course obvious clue was an incorrect tire size setting in my odometer. However, I double-checked to be sure it's correct:

I have (50-559) 26x1.95 tires, so I believe I should set it to 2089.

What else could cause the difference?

I have consulted this with a very experienced bicycle rider, but he mistakenly thinks it's because the Google Maps route doesn't always match the actual route. However, I spend some time adjusting the auto-calculated route, so it matches my actual route in about 99.5%.

¹ _{Assuming that my difference calculation formula is correct. Plus -- I don't know why divergence is higher in case of return trip. The only clue, that I have is that it takes much more backroads, turns, slow downs (while onward trip is riding on the streets nearly all of time), which causes, that travel time is longer in case of return trip even though it is shorter. But, that is a weak argument as divergence should be the same in both / all cases.}

Answer 1

I point out that your comment of

 I have (50-559) 26x1.95 tires I should set it to 2089.

Doesn't actually match your table, which says:

 50-559 (26"x1.9")     2089 mm
 54-559 (26"x2.0")     2114 mm

So the ETRTO number and the imperial measurement are not quite equal. Could be your circumference is closer to halfway between these, or 2101 mm

Downside, this would put your reading up by 0.5% making it even larger, and further from the map distance. Do the rolling distance measure repeatedly as others have suggested, for an accurate figure.

You can also consider putting your reader-magnet on the rear wheel, which should have less slip than the front one, and will follow a shorter path while you steer.

Answer 2

Google maps probably treats roads as not having any width, and corners as point turns. Unlike bends, if you zoom in you tend to see a sharp 90 degree change in line on a curve This will cost you the length of the arc you really take.

It certainly omits all the lane changes, obstacle avoidance and similar manoeuvres.

However there is some variation in tyres - particularly some of the anti-puncture tyres have a bit of extra thickness; inflation vs. weight on the wheel will also have an effect. So you're better off seeing the wheel circumference based on counting the wheel revolutions over an accurately-measured straight line.

Answer 3

First do a rolling measure while sitting on the bike to get the speedo calibrated to your actual setup. That's your method B

Then I suggest also getting a proper map and making sure google's scale is correct. They care about navigation much more than 5% errors on inner-city distances, so it might be that their map is less accurate than a proper cartographical one is. Elevation change could likewise make a small difference.

The small weaving motions you make while riding to stay balanced also add to the distance, especially for the front wheel. That's likely to be worse the slower you go and the more you weave around other road users. In the worst case, if you're weaving through stationary motorists at rush hour you could easily add 5% just for that. Similarly if you're travelling barely faster than walking on a crowded shared path.