Tire Size and Calibration Puzzle ?

So, these are the revolutions per mile specs.

Toyo Extensa A/S II 195 /60 R17 90H SL BSW TM: 792 rev/mi
Toyo Extensa A/S II 195/50R19 148690: 777 rev/mi

The difference is 2%, which means that your speed would be 2% off if it was calibrated for the other size.

 

But all other things won't be equal unless they are the same tire make and model. The diameter is probably measured without load, and revolutions per mile could vary according to how the tire handles the load depending on the specific design.

 

I have already answered your question regarding speedometer calibration. There is no deliberate mechanical calibration error (using the wrong rpm ratio) by Toyota but only a deliberate constant 2-mph calibration shift by the computer at all speeds, at least in my Gen 4 PHEV.

As for the LE/SE vs. the upper trims both having been calibrated correctly or not—I don't know.

 

That is very easy to answer.

When you get it, check your SE against the Google Maps navigation and see what it looks like. Or have someone helpful check their SE. Have someone helpful with the XSE/XSE Premium check theirs as well. If both are calibrated the same way, then the cars use a different rotation ratio. If the SE shows additionally 2% faster on the speedometer, then they use the same rotation ratio.

 

I actually checked it yesterday in my '24 XSE Premium because I kept seeing this 2 MPH number (or some people saying 2%) and I was curious. It's spot on even to the km/h compared to Google Maps. (This was with cruise control set to 65 MPH) It also seems that whenever I pass a speed radar it shows my speed as spot on to what the speedometer shows although that can be harder to gauge especially if there's other cars in front/behind

 

So, to confirm, when the cruise control set to 65 mph and the speedometer showing 65 mph, the actual speed on the Google Maps navigation is 63 mph, correct? If is interesting that it is also the same behavior in my Gen 4 PHEV. Note that this is not a 2% difference as some claim but a 2-mph difference—for example 22 mph, rather than 20.4 ≈ 20 mph, at 20 mph and 52 mph, not 51 mph, at 50 mph.

Now, we need someone to check with their Gen 5 LE/SE. I am guessing that it is the same; therefore, the computer is using a different rotation ratio in the speed calculations than the XSE/XSE Premium because of the 2%-smaller tire diameter.