Tire Size and Calibration Puzzle ?

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.

 

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.

 

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

 

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.

 

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.

 

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.

 

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

 

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)

 

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.

 

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

 

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.

 

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.

 

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.