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  1. #1
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    New bike computer... speed seems to be off?

    I just picked up a new Sigma 12.12 (my 1st bike computer) and put it on my 2011 Giant Roam. It works great and was easy to set up, however, the current/average speed seem to be off. My ride in to work is 4.83 miles and the bike computer hits that dead on. It took me just under 20 minutes for the ride. Based on that, my average speed should be 14.63 mph, right? Or at least my math says so? The sigma computer said i was 15.5 mph. Can anyone explain how the distance seems accurate but the average MPH seems off?

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    The computer is probably calculating your moving speed. When you are stopped at stop signs and red lights, your computer pauses, and waits for you to start again before restarting the clock.

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    But wouldn't that still be off? If the computer "pauses" (including the ride timer), shouldn't the average speed still be lower? I just want to make sure I am understanding how this works.

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    I don't know anything about that computer, but maybe the its pausing your moving time, when stopped, but not pausing your total elapsed time. Its all just a bit of a guess. Maybe somebody who owns that computer can answer your question better.

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    Senior Member Looigi's Avatar
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    Did you correctly set your wheel circumference?

    Distance with or without pausing is the same. If it takes you 20 minutes for the ride, and you stopped 3 times at traffic lights for a total of 3 minutes, then the moving time is 20-3=17 minutes. Same distance divided by less time equals higher avg speed.

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    Quote Originally Posted by Looigi View Post
    Did you correctly set your wheel circumference?

    Distance with or without pausing is the same. If it takes you 20 minutes for the ride, and you stopped 3 times at traffic lights for a total of 3 minutes, then the moving time is 20-3=17 minutes. Same distance divided by less time equals higher avg speed.
    Yeah, I've got the wheel size programmed in correctly. I guess what surprises me is that the distance seems to be correct and the "travel time", but the average speed doesn't calculate to what those 2 numbers equal. I would assume if I had the wrong wheel size set that the distance would be off.

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    Senior Member Looigi's Avatar
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    [QUOTE=Mastermind77;15615394...I guess what surprises me is that the distance seems to be correct and the "travel time", but the average speed doesn't calculate to what those 2 numbers equal.... [/QUOTE]

    Right. If it's using moving time, which is always less than elapsed (or travel) time, then the speed will be higher. Does the computer in question use moving time rather than elapsed time? Sometimes this is referred to as an auto-pause function and can be turned on or off.

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    I'll have to do some short test rides with it involving non stop riding vs. stop & go riding and see what I come up with. It does have the auto-pause feature with it so that may be influencing the outcome.

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    Quote Originally Posted by Mastermind77 View Post
    I would assume if I had the wrong wheel size set that the distance would be off.
    If the wheel size is wrong, the distance would be off.

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    Quote Originally Posted by Looigi View Post
    Did you correctly set your wheel circumference?

    Distance with or without pausing is the same. If it takes you 20 minutes for the ride, and you stopped 3 times at traffic lights for a total of 3 minutes, then the moving time is 20-3=17 minutes. Same distance divided by less time equals higher avg speed.
    This is confusing. He said the distance was correct. That indicates that the wheel circumference is set correctly.
    Last edited by njkayaker; 05-13-13 at 02:03 PM.

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    Quote Originally Posted by Mastermind77 View Post
    I just picked up a new Sigma 12.12 (my 1st bike computer) and put it on my 2011 Giant Roam. It works great and was easy to set up, however, the current/average speed seem to be off. My ride in to work is 4.83 miles and the bike computer hits that dead on. It took me just under 20 minutes for the ride. Based on that, my average speed should be 14.63 mph, right? Or at least my math says so? The sigma computer said i was 15.5 mph. Can anyone explain how the distance seems accurate but the average MPH seems off?
    "Just under 20 minutes" would mean 19.8 minutes to get 14.63 mph over 4.83 miles.

    15.5 mph over 4.83 miles is 18.7 minutes.

    That means you were stopped (or going very slowly) for 1.1 minutes.

    Quote Originally Posted by Mastermind77 View Post
    But wouldn't that still be off? If the computer "pauses" (including the ride timer), shouldn't the average speed still be lower? I just want to make sure I am understanding how this works.
    The clock time includes time spent stopped. The moving time excludes the stopped time. The distance is the same (it's one distance).

    That means the moving average (distance/(moving time)) is going to be higher than the basic average (distance/(moving+stopped time)).

    Using your numbers:

    15.5 mph = 4.83/18.7 > 4.83/(18.7+1.1) = 4.83/19.8 = 14.83 mph

    Quote Originally Posted by Mastermind77 View Post
    I'll have to do some short test rides with it involving non stop riding vs. stop & go riding and see what I come up with. It does have the auto-pause feature with it so that may be influencing the outcome.
    The "auto-pause" feature is common. It's easy to determine the basic average speed (the computer doesn't really need to give you that number).
    Last edited by njkayaker; 05-13-13 at 02:19 PM.

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    Alright, so I've checked a couple other rides, and the ride time, average speed, and distance are all calculating properly. Not sure what was up with that other ride, but looks like everything is good.

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