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Speedometer Correction, Can you explain why this formula works?

Discussion in 'The Okie Corral' started by Kevin108, Feb 22, 2013.

  1. Kevin108

    Kevin108 HADOKEN!

    Mar 2, 2005
    Virginia Beach, VA
    Two formulas for speedometer correction are presented "High-Performance Jeep Cherokee Builder's Guide."

    Formula 1:

    Old Tire Diameter / New Tire Diameter x New Gear Ration / Old Gear Ratio x Old Tooth Count = New Tooth Count

    Formula 2:

    (63360 / (MSOD x 3.14159) x Axle Ratio) / 74.5 = # of Teeth in Speedo Gear

    My algebra days are long behind me, so can I get someone to double-check my numbers here and explain to me how these formulas work?

    Since I was too lazy to pull my existing gear to start with, I used Formula 2 first after getting actual measurements on my tires. I have GPS verified the accuracy of my speedo with my old tires with a diameter of 28.25".

    (63360 / (28.25 x 3.14159) x 3.55) / 74.5 = # of Teeth in Speedo Gear
    28.25 x 3.14159 = 88.7499175
    63360 / 88.7499175 = 713.91615659811740106687986498692
    713.91615659811740106687986498692 x 3.55 = 2534.4023559233167737874235207036
    2534.4023559233167737874235207036 / 74.5 = 34.018823569440493607884879472531

    34 teeth and lots of decimal places!

    Now, back to Formula 1...

    I didn't change my gear ratio, meaning old / new would be 1, so I'll leave that part out.

    28.25 / 30.625 x 34 = New Tooth Count
    28.25 / 30.625 x 34 = 31.363265306122448979591836734694

    So I need to go all hillbilly and lose 3 teeth?


    But seriously, what is represented or determined by the numbers 63360 and 74.5?
  2. Steve0853


    Aug 18, 2003
    North Carolina

  3. wprebeck

    wprebeck Got quacks?

    Oct 20, 2002
    Mm..looks like heaven
    Don't know about the math, but I would advise you to not rely upon GPS as accurate. Its usually accurate to within a few meters - while great for figuring out where you are in a city, its not exactly mathematically accurate for determining speed.
  4. Two_Clicks


    Mar 7, 2010
    Southern Ohio
    You need the gear ratio for the speed the tire turns and the diameter for the distance the tire travels each turn, that is as simple as I can put it.
  5. syntaxerrorsix

    syntaxerrorsix Anti-Federalist CLM

    Oct 3, 2000
    Lakeland, FL.
    Garmin's specifications quote 0.1mph accuracy but due to signal degredation it could be as much as 0.5mph.

    Seeing as this is an average based on how many times the device pings the sat and reassess your position the only time GPS wouldn't be this accurate is if you were only driving a few feet (within it's error of location) per minute. At normal driving speeds the GPS is quite accurate.
    Last edited: Feb 22, 2013
  6. devildog2067


    Apr 20, 2005
    You need the velocity of the speedo gear, in # of teeth per second, to remain the same with the old tires/gears and new tires/gears. I wouldn't even really call it a "formula" it's just 3 ratios that you want to equal 1, then the bottom term on the right is cross-multiplied.

    The "tire diameter" and "gear ratio" terms give you the change in the angular velocity of the speedo cable ("how fast it's spinning" in terms of RPM). If the speedo cable spins 20% faster for a given speed, well than you want a speedo gear with 20% fewer teeth so that the speedo will count an unchanged number of teeth going by.

    Now lets talk about more important things: is this the old BAR Lucky Strike Honda car?

  7. kiole


    Feb 16, 2008
    GPS speed accuracy is usually very good. It's more accurate then your cars speedometer.
  8. wprebeck

    wprebeck Got quacks?

    Oct 20, 2002
    Mm..looks like heaven
    Well, for court purposes, I'm going to need to see the last time the GPS was calibrated and the user's training records on the device.

    Further, where did the OP state he used a Garmin device? What specific device is he using? Again, where are the calibration records for the device?

    I don't trust my speedometer at all - it reads at least 3mph over the actual speed I'm traveling, based on calibrated radar hits.
  9. SC Tiger

    SC Tiger Big, educated kitty cat!

    Aug 27, 2011
    South Carolina
    Here is how you do it:

    Drive at what your speedo calls 100 mph past a cop. When he pulls you over, ask him what speed he clocked you at. Now you know how far off your speedo is. :rofl::rofl:

    Formula 1 makes more sense to me because I don't know where the constants in formula 2 come from, and I don't know what MSOD stands for either. Either way the correction factor should be pretty simple to calculate.
  10. W

    W Book 'em, Spock

    Aug 15, 2000
    They don't even give those girls a chair to sit on?

    I'll offer them each a half a lap.
  11. syntaxerrorsix

    syntaxerrorsix Anti-Federalist CLM

    Oct 3, 2000
    Lakeland, FL.
    Not everything's about being a cop. Your statement is patently untrue across the spectrum of devices. As the unit itself doesn't measure your speed it won't need calibrating. I suppose you'll have to subpoena the satellite for that :wavey:
  12. Never Nervous

    Never Nervous

    Aug 18, 2009
    Metro ATL
    I agree with this. My GPS is spot on with my speedometer.

  13. DanaT

    DanaT Pharaoh

    GPS speeds I have seen are pretty damn accurate. Much more accurate than modern speedometers (electronic ones have a step function that are off more the fast you go) I have seen speedos read around 10-15% high at 125mph. They are showing 140ish and you are really going 125.
  14. Kevin108

    Kevin108 HADOKEN!

    Mar 2, 2005
    Virginia Beach, VA
    I used the GPS on my Droid X sitting next to my wife's Garmin Nuvi with cruise control on 55 mph as set by my gauge. All 3 devices displayed the same number.
  15. DanaT

    DanaT Pharaoh

    Cough cough cough bs cough cough cough.

    5.1.2. In the case of vehicles manufactured for sale in any country where imperial units are used, the speedometer shall also be marked in miles per hour (mph); the graduations shall be of 1, 2, 5 or 10 mph. The values of the speed shall be indicated on the dial at intervals not exceeding 20 mph and commencing at 10 or 20 mph. The indicated speed value intervals need not be uniform.

    5.3 The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified
    in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1)
    and the true speed (V2).

    0≤(V1– V2)≤0,1 V2+ 4 km/h

    Or maybe you have a super special car that was built in a special plant that makes cars that dont share common parts with other cars that have to meet worldwide requirements.
  16. Yeah, that's a lot of decimal places, but all meaningless. Look up the term ''significant digits''.
  17. pesticidal

    pesticidal Eh? CLM

    Jun 5, 2002
    North Dakota
    Their is no GPS calibration for speed. That's a fine-tuner for location.

  18. All speedos will be off as tires wear. If you put new tires on that are at all different in their circumference that will change the speedo accuracy.
  19. boozer


    Jul 24, 2007
    Hilltop, USA
    That's a lot of decimal places, here in Ohio they don't give you a ticket until you are four mph over the limit.
  20. My car is 16 years old and is off by up to 7 mph. It's more accurate at lower speeds but when it indicates 80mph I'm actually doing 73.

    Sensors can move out of calibration, that's why industrial facilities perform routine calibrations on them. My work performs quarterly calibrations on our nuclear density gauges.