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 Clydesdales/Athenas (200+ lb / 91+ kg) Looking to lose that spare tire? Ideal weight 200+? Frustrated being a large cyclist in a sport geared for the ultra-light? Learn about the bikes and parts that can take the abuse of a heavier cyclist, how to keep your body going while losing the weight, and get support from others who've been successful.

 04-16-12, 07:46 AM #1 ccrow Member Thread Starter     Join Date: Mar 2012 Location: Stillwater, OK Bikes: 05' Gary Fisher Nirvana Posts: 34 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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, 08:22 AM   #2
indyfabz
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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, 08:25 AM #3 TrojanHorse SuperGimp     Join Date: Mar 2010 Location: Whittier, CA Bikes: Specialized Roubaix Posts: 11,536 Mentioned: 53 Post(s) Tagged: 0 Thread(s) Quoted: 300 Post(s) 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, 08:40 AM #4 vesteroid Climbers Apprentice     Join Date: Feb 2012 Bikes: Posts: 1,608 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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, 09:22 AM #5 volosong Senior Member     Join Date: Apr 2011 Location: Antelope Valley, SoCal Bikes: Posts: 2,743 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 11 Post(s) 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, 09:42 AM #6 Seattle Forrest Senior Member     Join Date: Mar 2010 Location: Seattle, WA Bikes: Posts: 16,415 Mentioned: 12 Post(s) Tagged: 0 Thread(s) Quoted: 3447 Post(s) 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, 11:59 AM #7 avance Senior Member   Join Date: Mar 2012 Location: Maryville, IL Bikes: 2012 Specialized Roubaix Posts: 174 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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.