I installed an STI gauge cluster in 04' WRX and speedometer questions - Page 2
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This is a discussion on I installed an STI gauge cluster in 04' WRX and speedometer questions within the Interior Mods forums, part of the Tech & Modifying & General Repairs category; ^Yes it is best to use the same year but you still may have some dash lights. Probably the dccd ...

  1. #16
    Registered User ajpturbo's Avatar
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    ^Yes it is best to use the same year but you still may have some dash lights. Probably the dccd and the r. diff temp. But back to topic. I just called tacoma speed and they said nothing about a law but manufacturers are typically allowed to have up to a 3% error which isn't much, Honda's can have as much as a 5% error. It can be high or low. It does not have to be under! So your speedo can read slower than you are going. 3% error would only be 1.5 mph off at 50mph . I am 9% off and he said it's probably a combo of the wheel and tire size and electronic component differences between the two models.

    Nobody makes a speedo correction tool for these cars like hypertech and superchip so I'm stumped. My SCT tuner for my mustang have provisions to change rear end ratio and tire size to maintain an accurate speedo and my 08' civic SI can the speed adjustment to the dash and to the ecu by use of the Hondata Flashpro.

    The romraider opensource software that my car is tuned with also does not have any tables for speedometer correction.
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  3. #17
    zax
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    Quote Originally Posted by ajpturbo View Post
    It can be high or low. It does not have to be under! So your speedo can read slower than you are going. 3% error would only be 1.5 mph off at 50mph .
    I'm sorry but you are arguing a point that is incorrect! Speedometer laws are governed by UNECE regulations internationally (within United Nations jurisdiction). Federal law states that manufacturers must never produce a speedometer where the indicated speed is less than the actual vehicle speed... Source: UNECE - Transport Division - Vehicle Regulations - Addenda to the 1958 Agreement (Regs. 21 - 40). This is the case for the countries governed by UNECE -- United Nations Economic Commission for Europe - Wikipedia, the free encyclopedia
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  4. #18
    Registered User ajpturbo's Avatar
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    Tell that to tacoma speed I guess. I'm going off what he said.
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  5. #19
    zax
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    Well, I wouldn't necessarily trust the GPS reported speed either. Here's why:

    Civilian GPS devices are injected with approx. 3 meters of random error in any direction (this is government mandated). This means that even if the speed is sampled as slow as once every second, the speed reading may be as inaccurate as 3 meters/second = 6.8 miles per hour. I'm sure that the random error is not added in the same direction at all times, as this could potentially be compensated.

    EDIT:
    I realized that the speed likely sampled by the simplest method: take two points, measure the location and divide by the time taken between the two coordinates. Given this, the speed reading may be as inaccurate as (3+3) meters/second =13.6 miles per hour, though the likelihood of the error being at the maximum is very small. I'm willing to bet that the GPS coordinates are sampled at (perhaps) 10 times a second, then averaged and used as the coordinate... this is done twice and the speed is calculated from the time elapsed between the two points.
    Last edited by zax; 01-18-2011 at 11:14 AM.
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  6. #20
    Registered User economatic's Avatar
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    Quote Originally Posted by zax View Post
    Well, I wouldn't necessarily trust the GPS reported speed either. Here's why:

    Civilian GPS devices are injected with approx. 3 meters of random error in any direction (this is government mandated). This means that even if the speed is sampled as slow as once every second, the speed reading may be as inaccurate as 3 meters/second = 6.8 miles per hour. I'm sure that the random error is not added in the same direction at all times, as this could potentially be compensated.

    EDIT:
    I realized that the speed likely sampled by the simplest method: take two points, measure the location and divide by the time taken between the two coordinates. Given this, the speed reading may be as inaccurate as (3+3) meters/second =13.6 miles per hour, though the likelihood of the error being at the maximum is very small. I'm willing to bet that the GPS coordinates are sampled at (perhaps) 10 times a second, then averaged and used as the coordinate... this is done twice and the speed is calculated from the time elapsed between the two points.
    You're probably right about the way the GPS is calculating speed so to overcome that just reset the average speed measurement that the GPS likely calculates (some have speed and mileage stats you can reset). Factor in the Law of Large Numbers and you no longer have to worry about sampling error affecting accuracy of a speed measurement.

    Or...if you still fear the accuracy of GPS technology go old school and do the speed calculation with milemarkers.

  7. #21
    Registered User ajpturbo's Avatar
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    Damn Zax, you are making me want to hang on to my GPS. I ran a 12.3 at 124.7 mph in my civic and had my garmin in the car. It registered a top speed of 124, who knows maybe it was right on if it calculated in tenths. Mine must be a really good one.

    Sorry to be a douch but 3 meters per second is 6.71 mph not 6.8 your conversion factor must me slightly off. 39.37" in 1 meter or 2.54 cm is 1". Not sure what you used.

    Maybe there are tolerances used for speedometer calibrations but in my experience the automakers must not be utilizing the full margin of error becuase none of my cars have ever been off more than 1 or 2mph at most and that's going off gps and the radar machines on the side of the road that the pigs put up to slow you down.
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  8. #22
    zax
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    Quote Originally Posted by ajpturbo View Post
    Sorry to be a douch but 3 meters per second is 6.71 mph not 6.8 your conversion factor must me slightly off. 39.37" in 1 meter or 2.54 cm is 1". Not sure what you used.

    At least your honest. I actually did the conversion from 10 feet/second but then called it 3 m/s because I hate imperial units. Supposedly, government mandate puts a 10 feet cap on Civilian GPS accuracy. 10 feet/second is 6.81 MPH and that's where the number comes from. I didn't expect anyone else to put up a fuss about .1 mph! lmao!
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