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  1. #1
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    gps and bike computer differences

    I've noticed the speed and ride distance are different on my iphone compared to my bike computer. On the iphone i'm using gps lite and on the bike its a VDO computer. When I set up the computer i did not go with the recomended number for the tire size. I put the wheel and tire against a wall level. Used a two foot square on the floor touching the tire and measured the distance from the square to the wall. When i calculated the number it was different than the chart but it was the one i used. For example the easy ride i just did the bike computer says i went 13.75km and a max speed of 30.12km and the phone says I went 13.18km and a max speed of 38.4 km. Which one do i go by? or do i set my bike computer to the number on the chart for a 2.1 tire?

  2. #2
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    Go with the bike computer. The iPhones GPS is that accurate. Even a dedicated bike GPS won't be as accurate unless it has an ANT+ Speed Sensor.

  3. #3
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    I have no idea what you just described...certainly not the standard rollout method.

  4. #4
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    Quote Originally Posted by NateHawk View Post
    I have no idea what you just described...certainly not the standard rollout method.
    To find the exact circumfernce of a wheel. Diameter of the wheel in MM times pie. Taken right out of the instuction manual.

  5. #5
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    Quote Originally Posted by NateHawk View Post
    I have no idea what you just described...certainly not the standard rollout method.
    Diameter x pi = circumference. Circumference = rollout, correct?

  6. #6
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    Just went for a drive in the truck and the gps is bang on with distance and speed my my truck speedo.

  7. #7
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    Quote Originally Posted by Call_me_Tom View Post
    Go with the bike computer. The iPhones GPS is that accurate. Even a dedicated bike GPS won't be as accurate unless it has an ANT+ Speed Sensor.
    Typo, I meant "The iPhones GPS isn't that accurate.

  8. #8
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    Quote Originally Posted by 2 wheels and proud View Post
    Just went for a drive in the truck and the gps is bang on with distance and speed my my truck speedo.
    Your truck and your MTB are two different things. Let me know if your truck can do tight switch backs and see how well your GPS can pick that up.

  9. #9
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    Quote Originally Posted by scottvt View Post
    Diameter x pi = circumference. Circumference = rollout, correct?
    Diameter x pi is not the same as rollout because it does not account for tire pressure and rider weight, but thanks for the mathematical description of what you did. Dialect gets in the way sometimes.

  10. #10
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    The ride I did was on pavement that was almost prefectly straight and flat. It was just the path between houses to the 7 11. So it should be quite comparable to my drive down the highway. I weigh 140lbs and always run 40psi (which is high I know) so the tire squishing under my weight shouldn't be an issue. I also rechecked mesurements just now and everything checks out the same. Did the rollout method with me on the bike just to see and it was within a mm too.

  11. #11
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    Quote Originally Posted by 2 wheels and proud View Post
    The ride I did was on pavement that was almost prefectly straight and flat. It was just the path between houses to the 7 11. So it should be quite comparable to my drive down the highway. I weigh 140lbs and always run 40psi (which is high I know) so the tire squishing under my weight shouldn't be an issue. I also rechecked mesurements just now and everything checks out the same. Did the rollout method with me on the bike just to see and it was within a mm too.
    did you weight the bike with you + your gear? how much water to you carry? those mm's do add up, and it's generally recommended to do as many revolutions as you can with the longest tape you have.

    but even still, the discrepancy you describe in overall mileage between GPS and bike computer is to be expected. the speed is a little wonky, but that's a cell phone GPS for you. the apps have to heavily process the raw GPS data and they're not perfect.

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