Tire Size and Calibration Puzzle ?

Discussion in 'Gen 5 Prius Main Forum' started by Templeton, Nov 18, 2024.

  1. Templeton

    Templeton Junior Member

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    Not sure if anyone would know (or hazard a guess) regarding the answer to this question. Asking toyota directly is a total waste of time as they refuse to answer any technical questions. And dealers barely understand how cars work in general. Back on topic, on the trims of the prius, there are two tire sizes:

    the higher profile 195/60 R17 (LE/SE trims)
    and
    the lower profile 195/50 R19 (other trims)

    What is interesting to me is that the 19" tire/wheel combo is about 2% bigger in diameter (and circumference) than the 17" tire/wheel combo.

    I have read that toyota intentionally calibrates their speedometers to get drivers to drive a tiny bit slower than one's indicated speed. But, given that these two wheel types are themselves 2% different in size from one another, do you think that toyota calibrated the odometers, speedometers, and GOMs of the priuses with the bigger wheel tire/combo to be 2% different than the calibration on the priuses with the smaller wheel/tire combo (LE/SE trims)? Or vice-versa?

    Because, if not, it seems that one of the trims would get more accurate (or more inaccurate) distance, speed, and range numbers than the other. And, if there is only one type of calibration, is it more closely matched to the bigger wheel/tire combo, or is it more closely matched to the slightly smaller wheel/tire combo? Any thoughts?

    Interestingly, the same situation exists with different trims of the current rav4 Prime, the SE wheel/tire combo is about 2% smaller than the wheel/tire combo on the XSE trim.
     
  2. PriusCamper

    PriusCamper Senior Member

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    Prius used to be even higher in profile at 65 rather than 60. I took a chance on some ecopias that had a great price with a 60 profile and it makes a massive difference in handling, but calibration is off and car thinks I'm going 1-2mph faster than I am.

    I do know that @Mendel Leisk on here has a calculator to get the answer you want.

    I also know that 60 and 50 tire profiles are harder to find, less common and that if you get a destroyed tire while traveling finding a replacement tire is way harder, so switching to a different wheel with most common tire profile size would help address that.
     
  3. Templeton

    Templeton Junior Member

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    Thanks, I am familiar with those tire issues.

    But, even if I go to a different size completely but I still want to have the optimal calibration, I kind of don't know which direction (bigger tire/wheel combo or smaller tire/wheel combo) to go.
     
  4. Trollbait

    Trollbait It's a D&D thing

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    Haven't seen an answer as to whether Toyota installs separate calibrations for the two tire sizes.
    Over thousands of miles, it isn't going to amount to a serious difference if they used the same for either size. There is going to be the unavoidable error from the tires wearing muddling things, and installing another model of tire can introduce more even if it is the same size.

    Car companies only have two concerns when it comes to calibrating these meters. One is adhering to the margins of reading allowed by law, and the 2% difference falls within that. The other is avoiding customer complaint and legal action. Speedometers read high to avoid be hassled by people trying to pass their speeding onto the company. Odometers read low to avoided getting sued over claims of cheating on the warranty period.
     
  5. Winston Smith

    Winston Smith Member

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    If you have GPS on your cellular or radar detector, you should be able to tell readily whether you are running a bit slower than your speedo indicates. Mine (an LE with 17" wheels and Toyo tires) will set cruise at 72mph, indicate 71mph on the speedo and show 70mph on the detector.

    I'm thinking about going a bit taller on the sidewall.

    Tire Size Calculator
     
  6. Templeton

    Templeton Junior Member

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    What app do you use to get real time GPS speed info with your phone?
     
  7. Winston Smith

    Winston Smith Member

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    Someone smarter will know. I don't use cell phones. My kids have this program on their phones that shows a map and directions. I believe lots of those have that feature because they use GPS to locate the phone.
     
  8. rjparker

    rjparker Tu Humilde Sirviente

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    Toyota uses an accurate calc for miles and range which is displayed on the dash. They also have an accurate speed which is used for abs and other calcs. However the speed on the display shows ~3% fast to keep you under the speed limit (in theory).

    As a result they do have different calibrations set in the skid ecu at the minimum. When you buy that part, there are different part numbers corresponding to the vehicle trim.
     
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  9. Mendel Leisk

    Mendel Leisk Witness Leader

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    I'd suspect Toyota split the difference, used a theoretical OD that's an average of the two tire sizes. Maybe consider tire wear as well. It's worth noting: the car knows the speed more accurately, but adds a fudge factor for the speedometer, on purpose. The odometer should be as accurate as practical. If not they'll get their hands spanked.
     
  10. Templeton

    Templeton Junior Member

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    Interesting, so you think that the ECU for the LE/SE trims is programmed with the tire diameter that comes on those cars, and the ECU for the higher trims is programmed for the bigger tire diameter that comes on those cars? But, I am curious, other than the different part numbers for the ECUs, what makes you think that the ECUs consider the tire size difference.

    The skid ECU is the anti-lock brake controller, correct? And that ties in to the odometer, and GOM, and to the EV range and fuel economy estimates?
     
  11. rjparker

    rjparker Tu Humilde Sirviente

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    The speed and distance comes from the abs wheel speed sensors.
     
  12. Templeton

    Templeton Junior Member

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    Yes, but would the ABS not work as well if the wheel speed was off a couple of percent?

    Or, is there any info out there that confirms that the ECU is indeed calibrated differently for the different wheel sizes?
     
  13. darkstar3274

    darkstar3274 Junior Member

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    I have a 2024 XSE Premium that came with the 19inch wheels. I replaced them with the 17 inch rims from an SE Prime and added 20560R17 tires (same overall diameter as the original) I checked the miles on various trips (I mean various) with the phone GPS (all accurate)
     
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  14. Templeton

    Templeton Junior Member

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    Thanks, good info.

    But, I am curious, what app do you use to get real time GPS speed info with your phone?
     
    #14 Templeton, Nov 18, 2024
    Last edited: Nov 18, 2024
  15. darkstar3274

    darkstar3274 Junior Member

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    I just went with the distance. If it was 3.7 miles for a route, I followed the exact route and noted on the trip meter to the decimal. It worked on various routes of different lengths. (if this was your question). I have not monitored speed, though. Since distance is the great accumulator (acceleration and in turn, velocity/speed through time domain) it (distance) from the Google maps app ended up verifying for me that the odometer/trip meter was correct. I think my old Garmin if I recall correctly, gave speed - time to bring that out of retirement.
     
  16. PriusCamper

    PriusCamper Senior Member

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    Majority of OBD2 and travel map apps have that feature...
     
  17. Trollbait

    Trollbait It's a D&D thing

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    There is a vehicle speed sensor somewhere in the drive train, probably in the differential, reading a component's rpm. The ECU calculates vehicle speed from that using the gear ratios and tire sizes it was told to use.

    The ABS system calculates a speed from its sensors, but getting a precise speed isn't important. It is looking for a difference between its number and what the ECU has. A wheel locking up spins slower than the driveline speed. An error in calculations from tire size will apply to both calculations equally.

    For other systems working off the ABS sensors(traction and stability control, maybe ABS), tire size errors also isn't a problem. Tire size only could become an issue when there are differences in size between tires; like a mix of worn and new tires, or different models.

    Something to keep mind with tire size calculators is that most results are for tires with no load, or actually, the tire size as an ideal circle. Put them on a car, and that circle deforms as the weight load squishes the rubber. The actual difference between the sizes can be more or less, depending on the tire models and the inflation levels.

    Why is some important for you to know the exact speed? The other variables that influence fuel economy calculations and the GOM will over shadow a tire difference in time.
     
  18. Templeton

    Templeton Junior Member

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    Thanks for the detailed reply. No, the exact speed is not important. But if I slightly modify the tire size (i.e., get a different tire size than stock), I think that I would want to reduce (rather than increase) any speed/odometer/range/GOM number errors that might already be baked in the system. Or, so I would think?
     
  19. Trollbait

    Trollbait It's a D&D thing

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    As long as the diameter between the tires is close, you should be fine.
    The displayed speed is already likely on the high side. A car company got sued over odometers reading to high once, so most likely err on the side of to avoid that hassle. Weather, detours, and other changes will change the GOM.
     
  20. Mendel Leisk

    Mendel Leisk Witness Leader

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    The only "errors" should be the intentional high-reading speedometer. Again, the odometer should be (and VERY likely is) reasonably accurate, with either stock tire size. Obviously a little off with either, but in the ball park. Toyota does think about this stuff.