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Computer question re: tire circumference & mileage ridden...(4 posts)

Computer question re: tire circumference & mileage ridden...jtferraro
May 10, 2003 9:04 PM
Without realizing it initially, I discovered my new Vetta V100HR computer defaults to 700c X 20mm tires, which are the same as 26 X 2.0. Anyway, the other day I rode my bike, w/the standard 700c X 23mm tires, for what I thought was 19 miles. However, b/c the default setting was for 700C X 20mm (26 x 2.0) tires I know the 19 miles reading wasn't accurate. Can anybody tell me how far I REALLY rode the bike that evening?


re: Computer question re: tire circumference & mileage ridden...jiggs
May 11, 2003 9:36 AM
My guesss is that your 19 miles was more like 19.2 miles. If you multiply the diameter of the 23mm tire (26.75" actual diameter x 25.4mm/") 680mm X 3.14 you get 2135mm, the circumference of the 23 mm tire. I guess that the 20 mm is just 6 mm less in diameter or 673 diameter or 2113 mm circumference. So you travelled 2135 mm not 2113mm with each turn so divide 2135 by 2113 to get the percentage of error. Multipy the 19 miles x 1.01% and it is 19.2. I measured circumference once by putting a tie wrap around the tire, putting a spot of ketchup on and rolling the tire on the kitchen floor then measuring the distance between the tie wrap marks and it was right on. Of course this doesn't account for less than round, or flat bottom.
Sounds like you're a rocket scientist! ;-)jtferraro
May 11, 2003 5:27 PM
If I had known my real traveled mileage was SO close to 19 miles I wouldn't have even posted the question - DOH! Nonetheless, I do appreciate your work!


Measure or calibrate, it's up to youKerry
May 11, 2003 5:29 PM
Since there is no "standard" diameter for a given tire size, you've got two choices: measure or calibrate. You can measure the rollout of your wheels/tires (not somebody else's, yours!) and then correct the number in your computer. Do this with your bike fully weighted to be most accurate. Alternatively, you can ride a measured distance (10 miles or so) with accurate road signs, and correct from that. In parts of the midwest, we have "mile roads" which are surveyed to be on the mile and you can count on them. In NY state, any state highway has markers every 1/10 mile that can be added up. You need to go 10 miles or so to get the kind of accuracy you're looking for. Be careful about mileage markers on MUTs though, they were often done with the county Parks & Rec. pickup truck, and can easily be off by 2-3%.