# Tracking Mileage

• 04-16-12, 08:46 AM
ccrow
Tracking Mileage
So, i'm just getting started and I have an expensive (\$20) computer on my bike. I thought I had it set up correctly, followed all the instructions, measured the circumference of my tires with me on the bike. I compared it's calculated distance against Strava (Iphone GPS app) and they don't match up. I think they were off 3/10 over a 10.5 mile ride.

I'm sure I'm over scrutinizing at 3/10 difference in 10 miles, but I'm not sure which one would be correct.

On the mathematical side, I'm not completely sure I have my tire circumference correct. When I compared my measurement to the suggested measurement in the instructions for 700x32 tires mine was a little less. I figured it was do to compression of me being on it. Admittedly I don't know if a few mm's would be throwing it off that much.
• 04-16-12, 09:22 AM
indyfabz
Quote:

Originally Posted by ccrow
I'm sure I'm over scrutinizing at 3/10 difference in 10 miles, but I'm not sure which one would be correct.

So the difference is 1 mile vs. 1.03 miles. I don't think that's a meaningful difference or one you need to worry about.
• 04-16-12, 09:25 AM
TrojanHorse
No, it would be 10.5 vs 10.2 which is a bit significant.

Just go with your GPS app and use the computer for real time input while riding. Every now and then you'll get crazy results there too though, like a little quantum leap.
• 04-16-12, 09:40 AM
vesteroid
It's a 3% difference any way you slice it.

No more or less significant, just more or less miles different depending on the amount miles ridden.
• 04-16-12, 10:22 AM
volosong
An easy way to measure a wheel is put the bike on a sidewalk with the valve stem in the bottom position on a crack. Roll the bike forward until the valve stem is again in the bottom position. Then just measure with a tape measure, (and convert to cm/mm if you don't have a metric tape). Change in distance due to tire deformation of a rider's weight should not be significant.
• 04-16-12, 10:42 AM
Seattle Forrest
I don't think many people actually measure the circumference of their tire; it's usually a roll-out test where you put a drop of paint on the tire, then roll the wheel a few times, and take the average distance between spots on the road.

Your bike computer is almost surely more accurate than GPS, just based on the way each system works.
• 04-16-12, 12:59 PM
avance
I use multiple apps and a computer on my bike and all give me different results, but within 1-1.5% of each other. Strava rounds up.

I use Strava, Endomondo, and mapmyride(sometimes). Strava and endomondo tend to be closer together then my bike computer.