speedometer inaccuracy

Discussion in 'General Questions' started by pianoman8t8, Jan 30, 2007.

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  1. pianoman8t8

    pianoman8t8 Guest

    I got a speedometer, I'm not going o say who I got it from because that wouldn't be fair and it's irrelevant anyway...but anywho, i was driving along next to my uncle's car, my speedometer said 25 and he said his said 20, and we were going the same speed. So does this mean that whatever speed i'm going, I decreas it by 20% to make it actual speed? I want to see if i can't get an officer to radar me just for the fu of it, but what are the chances of that? just on observation and a thought. So would this also mean that the odometer would also say more milage than actual?


  2. etheric

    etheric Guest

    Feel free to mention the where the speedo came from, in the pursuit of knowledge nothing remains hidden.
    I think knowing the manufacturer would help us know what exactly you're dealing with.
    It may've been designed for a different sized wheel, or tire; which depending on what kind of speedo it is, could easily be reset.
    If you know of a cool cop who is interested in the bike, go for it.
    If not, seek out a school zone with one of those radar "you are breaking the law this much" displays.
    The odo is gonna be off too.
  3. It seems to me that speedometers are almost always off. It could be the car or it could be the speedometer on your bike. This is one reason I prefer digital speedometers, since you can calibrate it to the actual outer diameter of the tire. With cable drive bicycle speedometers its sometimes possible to swap the drive unit, they are each geared for whatever wheel size, but of course can't do anything about tire diameter. In any case, you just boosted the speed of your bike for very little cost :grin:
  4. Tom

    Tom Active Member

    Aug 4, 2006
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    maybe your friends speedo was off...
  5. nogoodnic42

    nogoodnic42 Guest

    I have found the best way to get an accurate speed reading is to use a GPS on the bike itself. I put the mechanical speedos on my bikes but they are by no means accurate, if I break 30mph the needle just starts pinging back and forth and running two bikes side by side I'll get two completely diffrent readings...Kelly
  6. retrophoto

    retrophoto Guest

    I had one of those digi things. When I put the magnet on the wheel, I put it in the wrong place. I was using the elec bike and it was nice to pick up that 2mph but alas now I know and it's nearly as much fun.
  7. MotoredTed

    MotoredTed Guest

    mio digiwalker

    I have a Digiwalker by Mio and it says My bike speedo is 2 miles per hour too fast,going around 30. Which means going around 28,if the gps is correct.

  8. eltatertoto

    eltatertoto Guest

  9. fetor56

    fetor56 Guest

    Are u sure u calibrated it properly...by properly i mean taking an accurate measurement of the wheelbase of the front tire after it has travelled one full revolution with u SITTING on the bike,then programming that into your computer?
    Last edited by a moderator: Jan 14, 2008
  10. Cookie

    Cookie Guest


    When I bought my speedom. it came with instructions for the different size wheel and how to so I followed the damrections and it seemded to work ok one of the local cops pulled me over for speeding but did not write me a ticket because I was on a bike and the judge would have thrown the ticket out. I do not go that fast any more the officer was right I was going to fast on a public road for a bike and a bikes breaks.

    I never have any luck with mechanical speedom they usualy go flying off the bike in pcs. but the dig works great.

  11. Smallwheels

    Smallwheels Guest

    Go Digital

    The digital bicycle speedometers are more accurate than any car speedometer if you calibrate them properly. My speedometer cost about $13.00 on sale at www.bikenashbar.com. They have different sales all the time so visit there often.

    The calibration process for this one was to just measure the distance of one full rotation of the wheel and enter the number in millimeters into the computer.

    GPS is not always accurate. It's good enough for dropping bombs on a particular location but not for measuring speed. Missiles do just fine with GPS because they just travel in straight lines above the ground. If one uses GPS to calculate speed the road must be perfectly flat, and straight as an arrow, otherwise any extra distance due to curves is not considered.
  12. Stryker

    Stryker Guest

    I am not sure if you understand GPS, The small Garmin 305 Cycling unit I use not only accurate to give or take 15 meters on your lat long, it also can give you your 3D location meaning Altitude. This all on GPS alone, most GPS also have internal barometers, for back up altitude, and WAAS that brings the accuracy to 3 meters. This you might not know is more accurate than the southwest 737 you fly on, which is not WAAS capable, (older technology). It will tell you exactly how fast you are going. But it will cost you.
    Oh and after i am done with my ride I down load it to motion base .com and replay my ride on an overlay google map. Speed, climb gradient, ect... and I can go back on my next ride and race myself.
    Soon I am going to see if I can beat my last ride using the boost bottle.
  13. kenspice

    kenspice Guest


    Hey Small Wheels,
    I have a Garmin60csx GPS. Put that name in your browser and look it up. You may get quite a surprise about the latest technology. As in the previous post, you will pay for it. You can buy it on sale now for about $275.00.
    With the new chip and all the goodies I always have 7 or 8 satalites locked in. They will update every .5 seconds and is probably as accurate as the cop's radar. I use mine for hunting and bicycling. They now run for 18 hours or more on 2 AA batteries. It keeps a record of every trip I do on it and my downloads to the computer. I used it on a trip to Hawaii and Japan and have a record on a map that goes right down to street address numbers of every foot of the way. Get one as they make biking and traveling MUCH more fun.
  14. LabRat63

    LabRat63 New Member

    Feb 3, 2008
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    I'm looking at possibly getting a Boost cell phone with a built in GPS. Anyone tried them or similar?
  15. Watcher05

    Watcher05 Member

    Jan 14, 2008
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    GPS's are accurate for speed!! The satalites have atomic clocks in them to operate correctly. I have what would be called one of the 'cheaper' portable ones around and I can be assured that I can find my little tiny crab traps everytime. Trilateration is what they use and have to be very accurate for them to work. Read: http://electronics.howstuffworks.com/gps.htm

    Last edited: Feb 17, 2008
  16. I got a cable speedometer. That needle sways here and there and lately hasn't worked at all but most important to me is the odometer. I took my bike on a highway just to calibrate my odometer staying far far away from other cars to the right I probably would have been stopped anyway and rode between two mile markers. I just went from one marker to the next one which is exactly supposedly 1/10th of a mile and my odometer read spot on.Then I walked off the highway.
    It's plastic inside with the needle looking pretty weak but the odometer part is geared so as long as the speedometer cable is turning my odometer is working.
    I never liked the electronic speedometer for I went on a long run with it one time and it didn't read. Then you can reset that odometer.
    I like a solid way of knowing how many miles you've traveled on your beloved. Then you can think about all those miles.
    Speed well I have one of those radar signs a block away.
    I already know what my top speed is.And I'll never go there again.
    I know my cruising speed is about 18 mph. Way under the get in trouble range.
    I say if you have to look at your speedometer,your going too fast.
  17. yeah I had one of those mechanical speedos on my bike. It was a total POS and I eventually took it off because I didn't like the look of it. I've stripped my bike down to the bare essentials and it looks a lot cooler.
  18. stringer

    stringer New Member

    Jun 15, 2008
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    which is better to get, the wired or wireless cyclometer?? :confused:
  19. kawasaki999

    kawasaki999 Member

    Jan 18, 2008
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    Here is the ticket.

    Attached Files:

  20. stringer

    stringer New Member

    Jun 15, 2008
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    is that directv? looks like the weather channel.