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Thread: Speedometer Calibration

  1. #11
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    Quote Originally Posted by jyoung8607 View Post
    Underlying that, the car still should have a pretty good idea of how fast it's going. It's been a very long time since I tried this, but I seem to recall both of my cars being pretty close to dead-on as compared to GPS.
    Correct.
    On My Octavia 2 I checked the actual speed of the car through hidden menu of Climatronic (https://www.youtube.com/watch?v=gIEE...bLAGQ&index=17 , channel 19, subchannel 0 (for km/h) or subchannel 1 (for Mph)) and the difference to GPS was only 1 km/h at 100 km/h, while the speedo was showing 106 km/h. So the car actually knows the actual speed, but it shows higher speed as Jason already described.
    Also this is very correct:
    Quote Originally Posted by jyoung8607 View Post
    The speedo will have been correctly calibrated as it left the factory: with the OEM wheels, with the OEM tires, and with OEM spec inflation. Any change here can affect accuracy. Not all tires and wheels of the same theoretical spec are exactly alike. Worn out tires will roll faster than new tires. Poorly inflated tires (temperatures are dropping!) will roll faster than properly inflated tires. Etc etc, and all sources of error are cumulative. It's not difficult to make a substantial difference. Spotting such differences is actually how indirect TPMS systems work. Some cars (I've personally witnessed this on a couple of Korean cars) are so picky they start having a bunch of false traction control and ABS engagements if you have a new tire and a worn tire on the same axle.
    Anyway, I don't have any experience with Jetta, but you should be able to get into hidden menu and then check for the actual speed. Maybe this will work (I suppose that your Jetta is based on Golf MK6, if it is still MK5, first video should work for you): https://www.youtube.com/watch?v=0JrYIvIljwo

  2. #12
    Verified VCDS User Spacewalker's Avatar
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    Take a VCDS, go to EGINE Measue Block ,and find a field with a Vechicle speed and compare this with your speedo.
    Did DIGITAl speed and "needle speed" is this same ?
    Also you can look in Cluster - Advanced Meas Block to check real wheel diameter.

    WHo said that GPS speed is right ?

    if you really want check car speed you should use tacho directly connect to the drive shaft
    HOT HOT HOT VW Touran 2005 with ACC HOT HOT HOT

  3. #13
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    Quote Originally Posted by jyoung8607 View Post
    There's definitely a small, positive error designed into the analog needle display.

    The speedo will have been correctly calibrated as it left the factory.

    Jason
    Jason, I come from a calibration world having run a standards lab and 8% is huge!!!

    Plus that 8% was the same when the car was a month old.

    Plus that same number is reported by other owners with the same model and series of car.

  4. #14
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    Quote Originally Posted by Spacewalker View Post
    Who said that GPS speed is right?
    Trust me, far more accurate than a speedo.

    Quote Originally Posted by Spacewalker View Post
    if you really want check car speed you should use tacho directly connect to the drive shaft
    And then that system would need to be calibrated. People, reading a display, even a digital display, means nothing unless the system is calibrated.

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  6. #15
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    Quote Originally Posted by Spacewalker View Post
    Take a VCDS, go to EGINE Measue Block ,and find a field with a Vechicle speed and compare this with your speedo.
    Did DIGITAl speed and "needle speed" is this same ?
    Also you can look in Cluster - Advanced Meas Block to check real wheel diameter.
    Of course, you can check actual speed also through VCDS, I forgot to mention this, because I always check it through Climatronic, since it is much safer than driving with laptop on your knees. If you have another person than it is a different story.

    Quote Originally Posted by Spacewalker View Post
    WHo said that GPS speed is right ?
    If you are driving on a a road without any descent/ascent and in straight line than I would say this is as correct as you can have it without any expensive measurement equipment.
    Just for example: we have traffic signs that measure and show your speed and in 90% there is no difference to GPS speed and the rest the difference is 1km/h.

  7. #16
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    Quote Originally Posted by mtl View Post
    Of course, you can check actual speed also through VCDS.
    There are only two speeds; "indicated" and "true" which we would hope are the same. But in many cases they are not. That is where calibration comes in, either in an adjustment or a calibration table.

    The VCDS can only report a speed based on the math within the module that handles the speed display. This will be determined by the tire rotation versus distance, the number of pulses (ABS wheel sensor or drive shaft sensor) and the math (including time) that then converts to a digital display of MPH or KPH.

  8. #17
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    Ideally, I wish VW AG would just give me a window in the program where I enter a number so as to get the correct speed displayed.

    Let us assume the ABS sensor ring has 72 teeth and that at 100 KPH, the wheel rotates 500 times per kilometer. That would be 36,000 pulses in 1 kilometer or in 36 seconds. Or 1000 pulses per second if the circuit is designed to update at that rate.

    All I would need to do is divide by 10 to get 100 KPH.

  9. #18
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    That would be perfect, but I guess there would be some legal consequences to that.
    This is also why speedos of all cars usually show cca. 5% more than the actual speed.

  10. #19
    Ross-Tech Employee Sebastian's Avatar
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    This should help you...

    http://forums.tdiclub.com/showthread...46#post3047646

    BTW: Don't calculate based on the analog gauges, if in doubt use the digital display as reference. The analog gauges are often enough not ideally calibrated (the motors that run them actually), so there may be some added inaccuracy. Also note that there are models which use secondary distance impulse number to calculate the distance travelled, so that may be off as well (meaning there may not be just 2 speeds you're looking at but 3)! In other words, double check not just the analog gauges but also the digital ones (and/or if not available the measuring values) as well as the displayed distance travelled. Test institutions use an external wheel with a defined circumference and calibrated speed sensors. They don't use any of the vehicle's own system, nor do they use GPS - yet for our use the GPS should be good enough.
    Sebastian @ Ross-Tech.com // VCDS Rookie since 2003

    »Nichts erweitert das eigene Wissen mehr,
    als die Meinung eines Andersdenkenden.«

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  12. #20
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    Well its maybe a little off-topic, but i think many Test institutions use these Racelogic units.

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