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Thread: Speedometer... does it lie?

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    Rookie viks783's Avatar
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    Default Speedometer... does it lie?

    I have heard from many people that the speed shown on the speedometer is not the most accurate. Is it true? I am not sure if they are speaking about the analog or digital but I was under impression that at least digital ones would be accurate!

    Please clarify. Or else I will be reading 100 and it would be something else. :P

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    Moderator The Monk's Avatar
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    Default Re: Speedometer... does it lie?

    Query Approved

    Yes, every bike has certain error. Percentage varies from model to model.
    Biking is not about what you have between your legs, its all about how well you use it!!!!!!!

    Give your details here if you want to help your fellow xBhpian stranded in your city

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    Default Re: Speedometer... does it lie?

    Reading of speedometer changes even if you are using a higher or lower profile tyre.. leave alone the built in error.

    Sent using Tapatalk 2

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    Default Re: Speedometer... does it lie?

    This is my theory on inaccurate readings of digital speedometers. Take it or leave it.

    Error is there but it's constant.

    Read this thoroughly if you want to understand what's wrong with speed reading :

    Assume this if you are riding at 120 on speed gun and your speedo meter is adding lets say 7 kms means it's showing 127...summary is that it's always going to show 127 (if actual is 120) not the 130, or any random thing...once we know that constant we can calculate actual speed..

    How does the speed get calculated ?
    This thing runs on pulse technology not on ancient magnetic wheel (seen in autos)...in this technology pulses are created at front wheel as it rotates...more rotations per second more are the pulse...than these pulses are counted inside speedo circuit and we get to see the speed on speedometer...its important to understand that this technology is working on pulse basis and not on the voltage basis.

    Then where is the error coming from ?
    To understand this we will have to consider an example...
    suppose front wheel creates 3 pulse per second at actual speed 20 kph
    so speedo calculates the speed like : 3 x 6.66 = 19.98 (rounding to 20)
    and at 4 pulses per second : 4 x 6.66 = 26.64 ( rounding to 27)
    so if u drive at any speed in between 20 and 26 it's always going to create 3 pulses only but after 26 kph it will be creating 4 pulses...
    meaning speedo will show 20 kph for any speed in between 20 to 26 kph and so forth...
    This example has assumed values but above scenario is true and that's where the error is coming from....in reality pulses count is greater than speed number meaning at the speed of 20 there must be more than 30 -40 pulses are getting created....
    If in future if those pulses are increased by the company then we will have more precise reading almost perfect like 99.99% but it never will be 100%...its the limit of technology...
    If you had studied the calculus > limits in school then you can easily understand what I'm saying

    Then if there's error problem with Digital Speedos then why are we using them?
    its simple because they are maintenance free and produce consistent result...for example if new bike is showing 130 at actual speed of 120 same bike or same model even after 20 years is going to show same 130 reading at actual speed of 120...
    So this little drawback of speed error is compensated with hug advantage it comes with...

    I found that error constant that I was talking about thanks to revtech and autocar who has provided speedo and true speeds
    speedo/true for my P180 UG4

    80/74
    100/93
    120/111

    if we divide them we get 1.081, 1.075 and 1.081 respectively..and now approximating them (1.081 + 1.075 + 1.081) / 3 = 1.079

    So constant is 1.079

    How to calculate actual speed?

    simple just divide speedo speed by our constant 1.079
    my bike has attained speed of 131 on speedo so the actual speed will be 131 / 1.079 = 121.4 kph
    this 131 wasn't the highest speed on my bike because next moment I ran into traffic signal
    I know somebody who had attained 137 on P180

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    Default Re: Speedometer... does it lie?

    on a lie-ter note.. speedometer is like a politician.

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    Default Re: Speedometer... does it lie?

    A high-end gps navigator can solve your problem ?

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    Default Re: Speedometer... does it lie?

    Wow. Nice analysis lockhrt999.
    TS - Yes, there are speedo errors at explained by honorable seniors above already.

    GPS bridges that gap, but again they're not 100% accurate.
    People say, Karizma ZMR and thr old P180 (fear the black version) have most accurate speedos. I have a ZMR and I tried verifying the claim.

    What I did - Took 2 mobiles with me. One android phone and one windows phone. I tracked my speed on both phones using gmaps and bing maps respectively. Parallel to this, I asked my room mate to track me on Google latitude.

    Reading obtained were (after 4 runs - posting max speed attained that day):
    Speedo - 98 kmph (couldn't go more due to a cow that came on road)
    Andoird GPS - 96 kmph
    Bing GPS - 96 kmph
    Google Latitude - 95 kmph (he said it flickered between 95 and 96 and finally settled to 95)

    Hence, the error varies bike to bike as well.

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    Default Re: Speedometer... does it lie?

    yesterday, i too checked my speedo error w.r.t. android based GPS app called "GPS Speed Graph"

    Speedo Reading: 104 KMPH

    GPS Reading: 91.8 KMPH

    but there's another thing, how accurate is mobile phone GPS...??

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    Default Re: Speedometer... does it lie?

    Quote Originally Posted by princesirohi View Post
    yesterday, i too checked my speedo error w.r.t. android based GPS app called "GPS Speed Graph"

    Speedo Reading: 104 KMPH

    GPS Reading: 91.8 KMPH

    but there's another thing, how accurate is mobile phone GPS...??
    I don't think you can rely on those GPS apps, if you want a good reliable GPS, its best to use the devices specifically designed for it. Even then it won't be 100% correct. But GPS speed trackers > GPS apps or those on phone(be it any phone).
    2013 R15(Sold) | 2014-?? Duke 390

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    Default Re: Speedometer... does it lie?

    Quote Originally Posted by Ezilkannan View Post
    I don't think you can rely on those GPS apps, if you want a good reliable GPS, its best to use the devices specifically designed for it. Even then it won't be 100% correct. But GPS speed trackers > GPS apps or those on phone(be it any phone).
    That is not completely correct. You are right in saying that GPS speed trackers are better than GPS apps but there is a reason for that. 1) Dedicated GPS speed trackers or GPS navigation devices have larger GPS antenna, for applications like speed tracking they trackers require a minimum number of GP satellites overhead.

    Now regardless of that if the conditions are not right, for example there are too many trees or building on the road side, or if its an over cast sky then ALL GPS devices are inaccurate, no matter how big their antennas.

    Corollary to this is that if you are using a phone app and you have open ground and clear skies and your phone battery is full then even phone apps are as accurate as GPS trackers.

    I found this first hand when I was using an application called Speedview. It was a clear night, I was on the open terrace of my building and nothing was obstructing my phone which was on air plane mode with GPS on. The application managed to track 12 GPS satellites, the reading was so accurate that it was mapping every FOOT STEP that I was taking. I am not surprised because even Google Maps has run on my phone with an accuracy of about 3 feet (1 meter). It was tracking my speed at 5 kmph which I know is correct because I know what 5kmph is having compared it to digital speedos. In anycase 5 kmph is too low a speed for the error to be significant so I know that the GPS was very near correct.

    All said and done I think if you want to end any argument on speed then there is no point in even talking about it if speed guns (the ones which police have and which use radar) are not involved. Otherwise all speed related arguments keep on going in circles with no end or conclusion ever.

    In modern bikes there are many many ways speed is measured. In many bikes speed measurement is done by the ABS unit where the perforated wheel of ABS is used to keep track of wheel rotation. This system is 100% digital and does not need any cable or anything. It has the same problem of "pulse" as described by lockhart above. So a larger ABS disc = more number of slots = more accurate speedo and vice versa. In some bikes the speed is measured by a sensor inside the crank case which directly measures the speed of rotation of the crankshaft and then adjusts it for the different gear ratios, final reduction & tyre radius digitally. This kind of speedo is most accurate but is affected by tyre size so if you change the tyre you will get erroneous reading.
    Divya Sharan and bbsrailfan like this.

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