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, with 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 it 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 in the XSE/XSE Premium because of the 2%-smaller tire diameter.

 

They could, but given that the main dash DOES actually know which tyres you were fitted with, to actually "split the difference" is conceptually more complex - it would have to know both its own tyre size, and the other tyre size.

You can tell it knows its own tyre size because it offers you the corresponding preset pressure in the TPWS menu.

It would seem very silly to tell the car the pressure for its tyres but withhold the rolling circumference.

I mean anything's possible - maybe something about the code structure means that the TPWS system is a million miles away from the speedometer and odometer - but it probably isn't.

 

What does the ECU need to know tire pressures for though? Before TPMS came along, there wasn't a way for the car of knowing it, and there wasn't a way to enter the value manually on past car models.

TPMS became mandatory to alert the driver to too low or too high pressures, cause many couldn't be bothered to check on their own. For that purpose, there is no reason for the system to report anything to the ECU beyond when to turn on a warning light. From the manufacturer's perspective, when put in the resources to tie it deeper into the software operating the car itself, and risk bugs from that, when not needed.

 

Have you not seen the UI? It offers 'set standard pressure', which shows you the pressure(s) from the manual for your standard size, or 'set current pressure'. There is no manual entry.

The only relevant point here is that it has already been given information about which tyre the car was fitted with.

 

No, I didn't word it clearly, but the speed Google Maps showing me was the exact speed the speedometer was showing me, even in km/h

(And to avoid people having to scroll up to the original post, this is a '24 Prime XSE Premium with stock 19" wheels and tires)

 

Yes this is what I used Rim & Tire Size Calculator. Custom Offsets - Wheel-Size.com

 

So for the curious folks (don't ask me as there are no economics to what I am doing much less anything else lol)...but am swapping the 205/60R17 Turanzas today for the Toyo 195/60R17 (I know theoretical 1.86% reduction in distance per revolution (need to multiply whatever I get with 0.9814 lol to get true measurement). Will report back in a few days on what actually happens with mpg, miles recorded, speed as there are other factors. All on the '24 Prius XSE Premium.

Again, to note: 205/60R17 on 17 stock rims for the '24 XSE Premium yielding in practice the same miles, speed and match GPS for speed and Gmaps for miles taking exact routes as Maps tells me to (various routes) as with the 195/50R19 stock tires and 19 stock rims. MPG improved a lot though.

Now to see what happens with the 195/60R17 Toyos on the 17 stock rims (which of course don't yield the same rolling radius)

 

more revs needed for same distance...i.e. less distance traveled than what will be shown on the trip meter/odometer/maps etc. Therefore multiply by 0.982 (all in theory of course-tire pressure, deflection also play a role) tire width plays a role in mpg as does tread pattern (no knowledge of this factor quantification but it is a factor)

 

If the car knew what tire it was fitted with, there wouldn't be a need for the user to select what tire size was equipped on that screen. Then that UI isn't evidence that there is communication between the TPMS subroutine and those responsible for the speedometer.

I know Europe is stricter about making mods to cars, are they also strict about keeping tire pressure set to factory? My 2016 Camry had 2 or 3 different factory tire sizes, but did not have such a display for setting TPMS pressure. There was just a button to push after putting the tires to factory pressures, or whatever you wanted.