My attempt to re-calibrate the speedometer

Discussion in 'Gen 3 Prius Accessories and Modifications' started by Val Rousseau, Apr 19, 2021.

  1. Val Rousseau

    Val Rousseau New Member

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    Hi everyone,

    Note to moderators: I already talked about this in a response to someone else's post, but then this made me think that it could be a good idea to post it here. Let me know if this is not proper.

    My speedometer's accuracy is pretty bad, which is annoying. At 64 mph (as measured on my GPS) it displays 69 mph. It is not a first-hand car, so I don't know what the previous owners did. But I believe it has the right tire size (195/65R15). I also believe from Carfax that the car wasn't intended to other countries.

    Whatever the reason for the inaccuracy is, I want to propose my own solution. I cannot confirm at this point that it works, as I haven't implemented it yet in my car, I only simulated it. But I hope to be able to try it soon, and will post an update about it.

    Here is the idea: I built and programmed a small circuit that intercepts the signal send to the speedometer, re-scales it, then forwards it to the speedometer. The reason I haven't installed the circuit yet in my car is that I want to check first that the levels of the TTL signal that it outputs are within the acceptable range of input voltage for the speedometer (I don't want to blow it out). I also need to find out if I can get a source of 5V from anywhere in the car in order to power the circuit. I will have to modify the circuit if only 12 V is available. These are just details. But from the tests that I made with my generator and my oscilloscope, it works properly.

    I know that I'm not the only one experiencing this problem, so if you are interested in the possible fix, check out my video of the test. I will make another video with the actual test on the car as soon as I find the time for it.
     
    #1 Val Rousseau, Apr 19, 2021
    Last edited: Apr 19, 2021
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  2. burebista

    burebista Member

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    Here in EU we have Regulation No 39 and we have those rules:

    The production shall be deemed to conform to this Regulation if the following relationship between the speed
    indicated on the display of the speedometer (V1) and the actual speed (V2) is observed:
    in the case of vehicles of categories M and N:
    0 ≤ (V1 – V2) ≤ 0,1 V2 + 6 km/h;
    in the case of vehicles of categories L3, L4 and L5:
    0 ≤ (V1 – V2) ≤ 0,1 V2 + 8 km/h;
    in the case of vehicles of categories L1 and L2:
    0 ≤ (V1 – V2) ≤ 0,1 V2 + 4 km/h.

    Long story short it's by design.
     
    #2 burebista, Apr 19, 2021
    Last edited: Apr 19, 2021
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  3. Val Rousseau

    Val Rousseau New Member

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    Yes, but anyway, I believe that my circuit can send a purposely "wrong" info to the speedometer, so that when the latter add its own wrong info it actually displays the correct speed.
     
  4. Mendel Leisk

    Mendel Leisk Senior Member

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    To make the car so it may under report speed could be illegal??

    this seems a Byzantine solution, to a questionable “problem”. If it bugs you, just speed a little?
     
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  5. Val Rousseau

    Val Rousseau New Member

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    I don't want to make it under evaluate the speed, I only want it to display the true speed!
     
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  6. Grit

    Grit Senior Member

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    i wouldn’t trust gps as an accurate measurement of speed. Find the nearest “your speed” electronic sign like a school and you’ll get a much more accurate speed measurement.
     
  7. Mendel Leisk

    Mendel Leisk Senior Member

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    Complete accuracy is never possible. Example:

    I once detailed a steel railing (in a public concourse) requiring rail spacing to not exceed 4”. I dutifully spec’d 4” spacing. General contractor called to explain: inspector will show up with close tolerance machined 4” dia ball, and if he can push it through the rail, anywhere, he will fail it.

    that was on my conscience for years lol. I think they just let it slide, or we got lucky with the tolerances.

    just food for though lol. But seriously, the mandate is ultimately, they don’t want people to be going FASTER than they think they’re going. Ever.
     
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  8. Grit

    Grit Senior Member

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    someone once told me on here “off topic” even though it was related to the exact idea.
     
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  9. SnowMexicanuck

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    If I go 50 km/h in my '15 Prius, the electronic speed signs will say 48-49 km/h.

    It's pretty much across ALL cars manufacturers to be slightly off. My '05 Cooper S doesn't show the same speed as the same electronic sign either. Honestly, I don't know why you're willing to mess with this. As someone else said, it's by design.

    If you mess with the speed sensor, it will probably code and unless you can mess with the ecu coding to fix it on the other end, that's way too much work for a 1-2 km/h difference. What tells you that your GPS isn't off?
     
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  10. RRxing

    RRxing Senior Member

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    ALL speedometers from ALL manufacturers are designed to read slightly (1-2 mph) higher the actual speed. Just another area where the lawyers want to minimize liability.
     
  11. ASRDogman

    ASRDogman Senior Member

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    My 2002 Town&Country Speedometer is 1mph slower.
     
  12. Val Rousseau

    Val Rousseau New Member

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    What is wrong with me wanting my speedometer to do what it is supposed to do, instead of what the government has decided it to do? My watch displays seconds, so it is legit to expect it to have an error no greater than one second. If my online bank shows that I have $3754.47 on my account, and I decide to withdraw it, I want to receive at least $3754.46, and it would be totally unacceptable to get only $3752.00. When I use my multimeter to measure a voltage, I want it to be accurate. Same thing when I use my oscilloscope. There is no reason for things to be different with a speedometer, and there is nothing wrong with refusing to be fooled by the government.[/QUOTE]

    At 64 mph, my speedometer indicates 69 mph. At 75, it indicate 81.

    I always get the same gap with any GPS. Also, my GPS agrees with the speed displayed by radars on the road. And, by the way, I have two Corvettes and a Mustang, and the 3 of them have a speedometer that agree perfectly with GPS and radars.
     
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  13. dbstoo

    dbstoo Active Member

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    In order to have the speedometer read 100% accurate, the diameter of the tire tread has to match the original design. When the post said that you think it's the right tire, that hints that you did not check this important parameter.

    As everyone knows, the tire diameter gets smaller as it wears. That means that the distance traveled per revolution will be less with tires that are close to replacing. It's against the law in most states to fiddle around with that number.

    When you buy tires you can look for the tread diameter and try to find one that matches your car's design. If you have 2 vettes and a stang that are all 100% accurate you are quite lucky, especially considering the propensity for adding odd sized aftermarket tires to muscle cars.
     
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  14. Val Rousseau

    Val Rousseau New Member

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    I checked since then, and I do have the correct size: 195-65-15.

    This is a myth that really needs to be busted: Brand new tires like mine, 195-65-15, have a diameter of 63.5 cm. When they are worn, they lose around 2.5 mm, which means that the diameter drops to 63 cm. The ratio new/worn is 63.5/63.0=1.008. This means that if the speedometer indicates an accurate speed of 60 mph when the tire are new, it will indicate 60 x 1.008 = 60.48 mph when they are worn. Less than 1 mph! I'm ok if it rounds it up to 61 mph.
     
  15. Elektroingenieur

    Elektroingenieur Senior Member

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    As I’ve mentioned before, there haven’t been government accuracy requirements for passenger car speedometers in the U.S. since 1982. The speedometer adjustment is a commercial decision by Toyota.
    A method of adjusting the speedometer reading that also causes the odometer to register less distance than actually driven would violate the federal and state criminal laws on odometer tampering, 49 U.S.C. § 32703 and (for example) La. R.S. 32:726.1.

    The more practical problem, on third-generation Prius cars, is that the speed pulse signal present at the combination meter’s SI input terminal isn’t used by the speedometer; it’s just distributed to the other ECUs connected to the +S output terminal. As the Repair Manual (more info) states: “This signal is not used for combination meter assembly [ . . . ] operation. Combination meter assembly components such as the speedometer operate using data received via CAN communication,” from the skid control ECU, which needs accurate wheel speed readings for ABS/VSC/TRAC to work properly.
     
  16. fuzzy1

    fuzzy1 Senior Member

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    The high readings are not mandated by "the government". It (or rather, some jurisdictions falling under that broad label) and separate liability issues only address low reading. If the speedometer read dead-nuts-on, there is no fuss.

    The high readings are driven by private corporations trying to build in some margin against getting themselves into troubles. Normal industry practices were established back in the old days when analog meters had significant accuracy problems. Today's digital measuring systems have narrower error bands, which has allowed the industry to shift to narrowed margins, but the common practice of building in some margin, persists.
    Try rechecking your calculation.

    Many tires start with about 10/32"nds of tread (some have even more), and remain legal down to 1/32" in a few states. While most set a limit of 2/32", certain dry southern states allow thinner.

    That means the tire can lose 9/32"nds of radius, or 18/32" of diameter, or more than 1.4cm of diameter, not the 0.5 cm of your claim.

    Note: Actual change of effective rolling circumference is a bit more complicated than this, and smaller than one would expect purely from changes of radius. Remember that the contact patch as flat, not circular, so the normal circle equations don't work out right.
     
    #16 fuzzy1, Apr 21, 2021
    Last edited: Apr 21, 2021
  17. Val Rousseau

    Val Rousseau New Member

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    If I really want to worn my tires so that they get slick, I can only lose 0.5 cm radius, thus 1 cm of diameter. That means that my original speed of 60 mph would become 60.96 mph. As I said, I'm ok if, with such a wear, my speedometer indicates 61 mph.

    I'm talking about a prius, not monster truck that can lose a diameter of 10 cm or so.
     
  18. ChapmanF

    ChapmanF Senior Member

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    My 195-65-15 tires are reportedly 831 revs/mile, so 31 cm (effective) radius, including 11/32" of original tread. So wear off 9/32" of that, or .71 cm, we're down to 30.29 cm. My 60 mph speed would be inflated to 61.48 at that point.

    Still sounds like something I could live with.
     
  19. jzchen

    jzchen Senior Member

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    Go bigger on the tires, 205/70R15? Went from 215/50R17 to 225/50R17 per @Airboss confirmation of no rubbing on our v and am much happier...

    moto g(7) power ?
     
    #19 jzchen, Apr 21, 2021
    Last edited: Apr 21, 2021
  20. dbstoo

    dbstoo Active Member

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    It's not really a myth if it's true, and your math says that it is true.

    BTW, it's the circumference that matters, not the diameter. I'm too tired
    to do the math. :)
     
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