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Thread: Speedo giving incorrect reading

  1. #61
    Join Date
    May 2009
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    Cowra, NSW
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    74
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    Just for curiosity on the way home from the Apollo Road run I checked the speedo against the 5k marked RTA just outside Yass. The Hume Highway is pretty flat & straight at this point.

    Tripmeter just ticked over to 387.8 and at the end of the marked 5k had gone past 392.9 that's 5.1 kilmotres - Not an accurate test as how much past the 392.9 had I travelled and how close to the 387.8 was I at the start. I could have been anything up to 200 metres out.

    Speedo out for sure - marked RTA distance, don't know!

    My Copilot satnav on occassions is very slow to find satellites, once or twice has lost signal - My new MIO and the old Destinator were almost instant, found satellites before the end of my driveway.

    A good few years ago I wondered about the errors as I drove door to door from Cowra to Forbes, the trip was shorter going than returning. I had to fill out a running sheet on the car there and back. Around 3 kilomtres differerence give or take a kilometres on different days and it was always further in a Falcon over a Holden. Early one morning I drove as much as possible on the wrong side of the road to see if I could get the distance to and from closer. Absolutely no change, my theory was that there were more left hand corners, so on the left side of the road I was in the shorter part of the arc. I also followed a fellow teacher to Forbes one morning, he in a Falcon and I was driving a Commordore, both cruise controls and speedo's at 100 kph. I had to back the commordore off to approx 97 kph on the speedo to stop running up his bum and keep pace behind.

    Point is we know the speedo's are inaccurate, so we just have to live with it.

    I had heard once that it was done on purpose by the manufacturers, would you believe, to increase fuel economy. That was back in the days of the fuel crisis. I don't hold much credence in that theory though.

  2. #62

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    Speedo accuracy is a tough one, as tyre condition/size changes a fair bit through wear and pressures. Imagine a new tyre with a full 11mm tread depth, and then one with 2mm depth. There is 18mm of diameter there already. I remember calculating once on my rallycar (for the Terratrip accuracy) and there is about 4% in tread depth alone for the 195/60s that I ran from a new tyre (which started at 7mm depth) at the start of an event to the end of the event.

  3. #63
    Join Date
    Nov 2008
    Location
    Craigieburn
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    50
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    What I've found is that at 100K/h comparing speedo and ECU reported speed there is a minimum 5K error that varies up to a maximum of 8K on 225/45. on 215/45 you're prob at 9K, and 225/40 11K+

    It doesn't seem to be consistent enough to pin down a value beyond a 5K @ 100 minimum. Whatever the error is, on mine, will be a consistent K error across the 30-110 speed range. Error varies from day to day and spends more time at the high size of the range than the 5K minimum.

    So that is to say that if you correct the needle, 5K @ 100 would be a safe margin.

    This might possibly be achieved by flashing the ECU with a new tire size variable : ScanXL software reports a PID "MAZDA.ENGINE.TIRESIZE" - this value is rev/Km. Without being certain of functionality, this suggests adjustjment could be made to the ECU's tiresize variable, optimizing for your aftermarket tiresize and/or correcting the 5K safe margin.

    Correcting the needle error so would have your ECU underreporting slightly, and I suppose this is where you would want an accurate GPS and reference speed trap to work with.

    I am wondering what the legalities might be of adjusting so.

  4. #64
    Join Date
    Jan 2010
    Location
    Durack, QLD
    Age
    40
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    495

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    Thank you people!!
    I have been driving around for the last 5 months absolutly confused about why everyone else on the road was going about 5+ ks faster then me around town... It was sending me crazy cause I dont have a GPS so I couldnt test it.
    Saying that though I have been pulled over for doing 80 in a 60.. And I am pretty sure I was doing 80. So I am going to be safe and just go by my speedo reading :-P

  5. #65

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    japanese cars tend to read higher than euro cars in my experience

    get a tuner to fix it so its spot on

    on the brightside.....you wont get nabbed for speeding.

  6. #66
    Join Date
    Dec 2008
    Location
    Sydney
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    The ECU reads the speed pretty accurately, well so I have found between the dashhawk and GPS, its the actual gauge that reads somewhere in the order of ~10% higher, most likely as a bit of a legal buffer.

  7. #67
    Join Date
    Jan 2010
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    Durack, QLD
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    I'd put a theory together that Wardski had a lead foot and he got the speedo changed to compensate so he didn't get caught.. Now I know the second part of that theory isn't true :-p

  8. #68
    Join Date
    Dec 2008
    Location
    Carindale
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    37
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    gps reads 7 kms less thn my speedo...

  9. #69
    Join Date
    Nov 2008
    Location
    Geelong, Victoria
    Age
    56
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    121

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    The speedo is out about 8%. There is a kit available called a speed corrector kit. This kit alters the speedometer signal up or down from 0% to 99% of the original signal. With this improved model, the input setup selection can be automatically selected and it also features an LED indicator to show when the input signal is being received.... Only problem is that im fairly sure that our car uses all 4 inputs from the sensors near the wheels (you can see the cable and tubing if you take the wheel off.. its on the top). Also, you would need 4 of these "kits" and they are about $50 each. Also, if we fitted these kits would it upset the anti lock braking and traction control????? It would only be 8% different... probably enough to cause problems to the computer.. ie it might get "told" the wrong digital inputs.... I actually bought this kit over a year ago... did not realise that the car uses 4 inputs to get its speedo reading. I could be wrong but im not confident enough to buy 3 more kits and start hacking into the MPS's wiring.... I will advise when I get any more info!!

  10. #70
    Join Date
    Dec 2008
    Location
    Carindale
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    intresting

  11. #71
    Join Date
    Nov 2008
    Location
    Geelong, Victoria
    Age
    56
    Posts
    121

    Smile Solution to the dreaded speedo being inaccurate!

    Hi guys. Ive been frying my brain for months trying to work out a solution to our speedo's being out. I found a very easy solution!

    You need to obviously remove the speedo from the car.. The top plastic bit above the speedo will just clip off.
    You need to remove the drivers side trim bits and there are also a few screws that hold the bottom of the speedo.. Im sure you can work out this bit.

    Once you have the speedo out you need to remove the 4 outermost screws to remove the black rings. Remove the black rings. Then, unclip the perspex part and remove it. Take your time, it is not a race!

    Put the speedo back in your car with the perspex and black rings still REMOVED.

    Put a GPS or equivalent on the windcreeen and go for a drive.
    All you have to do is SLIGHTLY adjust the needle. Don't try to remove it (like I did!) cos then it might go a bit jerky.

    It took me about a hour to calibrate it.

    I firstly got the car to 100klm acording to the GPS and then adjusted the speedo accordingly.

    It takes quite a few goes to get it spot on, but now my speedo is about 2% out at the lower speeds (like 40klm).

    At 100klm, 90, 80 it is spot on!

    Now I just have to get used to it being accurate and not adding those extra klms on it.

    Hope this helps!

    Greg.

  12. #72

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    Or you can go up one size in the diameter or your rims, but keep the same profile and width of your tyres.

    This will bring you to within about 1% of a true 100km/h.

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