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)