Speedometer reads fast

Discussion in 'Technical Q&A' started by CarlH, Apr 6, 2004.

  1. CarlH

    CarlH Karting

    Feb 5, 2004
    Full Name:
    Carl H.
    Passed one of those radar trailers which show your speed today. I always suspected that the speedometer was reading too fast, and the radar indicated speed which was 7 MPH slower than the speedometer. Bad news is the bad reading - Good news is - I'm driving too slow. Any clues on adjustment/calibration of the unit? I have to figure that if I am reading too fast, that I might also be registering extra mileage - not a good thing.
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  3. Husker

    Husker F1 World Champ
    Silver Subscribed

    Dec 31, 2003
    western hemisphere
    My speedometer reads too fast, too. It's set on 10mph before I even start the car!
  4. Smiles

    Smiles F1 World Champ
    Lifetime Rossa Owner

    Nov 20, 2003
    Pittsburgh, PA
    Full Name:
    Matt F
    I don't know what Ferrari's specifications for speedometer tolerance are, but here's a service bulletin that explains BMW's. Interestingly, a higher-than-actual reading does NOT have an effect on the odometer.


    Instruments Product Engineering
    SUBJECT: Maximum Permissible Speedometer Error
    MODELS: All
    General Information: A certain amount of "speedometer advance" is necessary to compensate for negative tolerances in tire diameter, electronic controls, and other factors. This advance reduces the risk of the speedometer displaying a speed that is less than the vehicle's actual speed. The amount of speedometer advance can be calculated by inputting a frequency (using the DIS Tester, "Kombi Test Schedule") that corresponds to a given speed and noting the speed that is indicated by the speedometer. The maximum permitted speedometer advance is 10% of the actual (input) speed plus 2.4 mph.
    Actual Speed = 50 mph
    50 mph x 10% = 5 mph; 5 mph + 2.4 mph = 7.4 mph
    Therefore, the permissible displayed speed is 50 to 57.4 mph.
    (Note: The displayed speed must never be less than the actual speed).
    No repairs should be attempted if the speedometer advance is within 10% + 2.4 mph.
    NOTE: The amount of speedometer advance has no effect on recording of accumulated mileage in the odometer display. The odometer records total mileage digitally and does not incorporate any "advance" tolerances.
    Warranty Status: Information Only.
    SI Maximum Permissible Speedometer Error BMW AG * TIS 09.05.2002 19:32
    Issue status (03/2001) Valid only until next CD is issued
  5. timthetooth

    timthetooth Rookie

    Dec 18, 2003
    I have an 1984 400i auto. The speedo was inaccurate as you have found yours to be. Once you have the speedo out of the car, [about 30 seconds on a 400] remove the outer case and you will see a small circuit board with a set screw roughly in the middle. This screw adjusts the calibration of the dial. I did mine by trial and error, following articulated lorries down the motorway! They are limited to 56 MPH. It took about 6 or 7 tries to get it right, I am a patient type of guy! I had the chance after that to be timed by radar and was pleased to see 140 MPH exactly as shown by the speedo.
    P.S. Have seen 150 MPH since on a slight down hill run in France, I think the guy I rapidly caught in a 360 was a little shocked [as he was running a steady 130 MPH] to see the old girl in his rear view mirror!
  6. JOEL D.

    JOEL D. Karting

    Nov 2, 2003
    Boston, MA
    Full Name:
    Joel DeLuca
    Speedo is not is just optimistic!!
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  8. CarlH

    CarlH Karting

    Feb 5, 2004
    Full Name:
    Carl H.
    Nice to know I have company. I need to check to see if I am a constant 7mph off at all speeds, as with an offset, or a calibration issue which is off by a percent of speed. I suspect it is the former. Probably should go look to see if the car is doing 7 mph parked in the garage. Will try the adjustment posted in the thred - thanks!

    Change topic - it never ends.....Today - Sunny day afternoon drive, roof off.....suddenly ZZ top gets drowned out by the sound of a blown exhaust...mine. Nuts.
  9. Mr Iceman

    Mr Iceman Karting

    Mar 3, 2004
    Canterbury, England
    Full Name:
    Hi, Its my understanding that by law (in the UK at least) that manufactures have to make there speedos accurate to within +10% -0%, so most manufacturers set it to read in the +5% to +8% margin, I your car is within these tolerences then your same as every body else.

  10. Mr Iceman

    Mr Iceman Karting

    Mar 3, 2004
    Canterbury, England
    Full Name:
    Even if your speedo reads 10mph when sitting still, it doesn't mean its over reading all the way up the scale, my speedo is pegged at the lower end at 15mph, ie it wont even start reading until I Reach an indicated speed of 15mph or more. eah instrument has a calibrated range look at your oil pressre and temperature gauges, they don't shft off the bottom peg until quite a high value is reached.

    As an aside the Airspeed indicator on the planes I fly doesn't start to move until 40 Knots.

    Hope this is helpfull
  11. Doc

    Doc Formula Junior

    Sep 13, 2001
    Latham, New York
    Full Name:
    Bill Van Dyne
    Mine also reads fast. Does this mean that extra miles are being clocked on the odometer also?
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  13. Brian C. Stradale

    Brian C. Stradale F1 Rookie
    Lifetime Rossa

    Mar 17, 2002
    Dallas, TX, USA
    Generally speaking, yes. Use the mileage markers along side the road to check. They typically measure a bit less than 10% fast. But change to lower-profile tires can elevate that further.
  14. jbcottrell

    jbcottrell Rookie

    Apr 8, 2004
    I drove an 84 308 at about 60 MPH (guaged by the seat of my pants) however the speedometer indicates 100MPH. What was really fun was the freeway at 80-85 with an indicated 140-150! I needed a camera for that one. Has anybody had such experiance. I don't think tire diameter could make that big a difference?
  15. docapl

    docapl Formula Junior
    Silver Subscribed

    Apr 26, 2002
    SF Bay Area
    Full Name:
    Whenever I drive past these things they just flash SLOW DOWN or TOO FAST at me. :)
  16. LouB

    LouB Formula 3

    Apr 15, 2001
    FL, OR
    I understand EU specs require speedos to "never" read low therefore they are set to read 5-10% high. Not sure what our DOT specs require but suspect about the same. Get a handhald GPS and check speedo error across the speed band or stopwatch time mile markers while holding different speeds.
  17. Steve

    Steve Formula Junior

    Nov 1, 2003
    Seattle, WA
    Full Name:
    Steve Jenkins
    I like to make my hand-held GPS unit with me in my cars (a Garmin III). This allows me to check the accuracy of the speedo (and help me when I miss an exit). The 512TR speedo is DEAD on, while both the 355 and F40 are about 5 MPH optimistic. Just learn what the variation is for your car(s) and adjust in your head on the fly :)

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