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How accurate is your speedo? + Maths & regear questions/answers

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  • How accurate is your speedo? + Maths & regear questions/answers

    Hi, in a bid to find out how fast I'm going and check real MPG I need to start at the beginning and work from there so bear with me, this might be long one!;

    I am under the impression that a 1:4.875 + 31" tyre combo is 'standard' or at least the most issued combination? If not please correct me.

    If this is the case then the standard speedo in KMs should be pretty accurate so I need to know whether the speedo conversion plates fitted on import have the numbers in the right place (you never know with cheapo face plates!)?

    If it is then we are all able to have a good place to start when regearing or changing tyre sizes to correct our speed with regard to what the speedo says.

    Now comes the maths for regearing or tyre size changing (effectively the same thing but in different places).

    I have two regears on my system as not only are my tyres 33" but my diff ratio is 1:4.556 instead of 1:4.875.

    I'll start with gear ratio increase.
    Stock Ratio divided my Current Ratio x 100 = 7% difference
    so take my speed and add 7% for the correct speed, ie my speedo says 50MPH but I'm really doing 53.5MPH.
    Please check my maths.

    Now, the same with the tyres
    Newer tyre size of 33" divided by Stock size of 31" = 6.5% difference
    so again add 6.45% to my already modified speed and I get 56.95MPH.
    Again, please check my maths.

    If that's correct my speedo is as follows

    Speedo -> Actual Speed
    10 -> 11.39
    20 -> 22.78
    30 -> 34.17
    40 -> 45.56
    50 -> 56.95
    60 -> 68.34
    70 -> 91.12


    This can now be used to work out the correct MPG.

    Using the figures previously worked out plus some extras and a few other simple calculations I get the following information for MPG

    Reported KMs = 424 -> Actual KMs = 482.96
    Actual KMs to Miles = 300.1
    Litres used = 59
    Litres to Gallons = 12.98 Gallons
    so
    300.1 divided by 12.98 = 23.12 MPG

    Cost per Mile if you want it is worked out as;
    Cost per fill divided by mileage for that fill
    In my case £82.54 divided by 300.1 = £0.28 p/m
    (to check it you can use the figures to work out that it was £1.40/L to buy)




    Lots of information there, most likely not in a very easily read format but I'll tidy it up a bit as I go.

    My formulae are all in an easy to use Excel file so once the maths have been proven I'll upload it and everyone can use it and you only need 7 things;
    Stock Ratio, New Ratio, Stock Tyre Size, New Tyre Size, Reported KMs, Your Fuel Receipt with Amnount of fuel bought and cost of transaction.
    This will give you Actual KMs, Actual Miles, Gallons, MPG, £/M and £/L




    Also, usual things apply, if I'm wrong then sorry, but that's why I've asked for help and secondly, real world changes in tyre size need to be accountaed for, this is a straight text book setup, if I measure the actual circumference of my tyres (which I will do for accuracy) the results will be slightly different.


    Cheers,



    Ray.
    I've got a plan and it's as hot as my pants!

  • #2
    dakota digital sell a magic box for around $70 that can be used to re calibrate the speed sensor signal. works a treat on surf sensors and it corrects the speedo reading. can be calibrated for all tyre sizes and diff ratios.

    would save you a rainforest worth of paper on your maths and it wont give you a sore head either

    Comment


    • #3
      Check your speedo vs. a GPS to see how accurate your gauge is, GPS is always right so long as you are travelling a constant speed. I found my speedo was almost dead on accurate after upgrading to 265/75/16 tires, it read about 8km/h slow at 100km/h prior.

      If your speedo is out, so is your odometer.
      '96 Toyota Hilux Surf SSR-G - RIP

      Comment


      • #4
        Originally posted by IanB2 View Post
        If your speedo is out, so is your odometer.
        That chart accounts for that too, I'll update the details tonight.
        I've got a plan and it's as hot as my pants!

        Comment


        • #5
          Originally posted by gary-r View Post
          dakota digital sell a magic box for around $70 that can be used to re calibrate the speed sensor signal. works a treat on surf sensors and it corrects the speedo reading. can be calibrated for all tyre sizes and diff ratios.

          would save you a rainforest worth of paper on your maths and it wont give you a sore head either
          It would have to be seriously magic as I have a pre-facelift 2nd Gen and therefore my speedo etc. is all mechanical.
          I've got a plan and it's as hot as my pants!

          Comment


          • #6
            If you want more accuracy, then do not use the diameter stated for the tyre. Measure the LWR (Loaded Wheel Radius) instead. This is the measurement from bottom of tyre to centre of the hub. Multiply that by two to get a more accurate diameter.

            Nev

            Comment


            • #7
              Originally posted by NiftyNev View Post
              If you want more accuracy, then do not use the diameter stated for the tyre. Measure the LWR (Loaded Wheel Radius) instead. This is the measurement from bottom of tyre to centre of the hub. Multiply that by two to get a more accurate diameter.

              Nev
              Yeah, I was just proving the figures before getting too pedantic about it but that definitely the best way as tyre sizes vary greatly between models and manufacturers.
              I've got a plan and it's as hot as my pants!

              Comment


              • #8
                all standard horribly accurate according to GPS - I have to watch out for the camaras.

                Comment


                • #9
                  By standard I take it you mean 4.875 ratio and 31" tyres?
                  (Just to help confirm my original statement or not?)
                  I've got a plan and it's as hot as my pants!

                  Comment


                  • #10
                    Err standard wheel and tyres, 15".

                    Ratio no idea standard I guess.

                    Comment


                    • #11
                      Originally posted by lord_flashart View Post
                      It would have to be seriously magic as I have a pre-facelift 2nd Gen and therefore my speedo etc. is all mechanical.
                      oops - never noticed it was a pre facelift - your'e right - it would have to be a magic box to work on a cable drive speedo.

                      it does work well on the later sensor set up though - i run 3.42 diffs with 33" tyres and the speedo is bang on ( checked by a friendly local traffic cop)

                      it is a good way to save on speeding tickets and camera flashes when you have a non standard set-up.

                      Comment


                      • #12
                        I'm pretty sure standard tyres are 265/70x15 which are about 29.6 inch. Some people here have mentioned 31's being an optional upgrade.
                        Last edited by wishbone; 13 November 2011, 22:06.

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