Tire Size and Calibration Puzzle ?

Sure would be nice if Toyota Techstream had a way to change how those numbers are processed based on tire and wheel size all the way down to amount of wear on the tread. Of course most people just drive their car and don't want to worry about any of that.

 

Yeah... Gen5 Prius designers made it real hard to have plenty of options for tire shopping. But maybe in time the market will catch up to demand for odd sizes?

 

The only vehicle speed pulse comes from the skid ecu which calculates it from the abs sensors. The speed on the display is adjusted by another ecu and is transmitted along with accurate odo data to the combination meter (display) using the can bus.

Many decades ago, a transmission based mechanical cable or sensor was used for the speedometer.

In this simplified Toyota dealer description, the ECU being discussed is the skid/abs ecu.
What Is A Wheel Speed Sensor And Why Is It Important? | Toyota Parts Center Blog

 

For Android phones Ulysse speedometer app.

 

My 2021 Prius Prime Limited with OEM Dunlop tires shows exactly 2 MPH higher at all speeds on the speedometer than the actual speed.

One of the best ways to measure the actual speed is the Google Maps navigation app on your phone, which uses the GPS.

 

Yes my New Zealand 2019 model is about 4kph higher than actual speed, so corresponds to your 2MPH higher. Mine is not on OEM tires but same size as OEM. Makes it easy as I just drive on cruise control set to 5kmh higher than the posted speed limit.

 

At highway speeds, Prius Prime Speedometer reading 3% higher than Waze on OEM 19”.

 

yes I only got summer tires Bridgestone Turanza T005 in 205/60R17...so far still ok even in sub-50 and low 40s...unsure how this will be once it gets into 30s consistently

 

used Galileo GPS speedometer app today speed exactly matches the car display-low speed to high speed - it kept up with the displayed speed no issues on the speedo

17 inch wheels from SE with 205/60R17 Turanza on XSE Premium...

 

The roadside radar speed displays are more accurate than gps.

 

Both the GPS and roadside radar displays always show exactly 2 mph lower than my 2021 Prius Prime Limited speedometer at all speeds.

If you have a good phone with good GPS reception and enable accurate location, it should easily be accurate within a fraction of an mph.

 

Those are rounded, though GPS could too depending on device and app. i also wouldn't put it past a municipality setting theirs to display high.

If you want precision with whole number digital displays, best to use km.

 

I wouldn’t go any smaller diameter than the stock 195/60/17’s because my SE is already really low to the ground.

 

Going to 205/60R17 yields a theoretical OD of 26.7". IIRC that's identical to the stock 19" tire's OD?

This site is good for checking:

Tire Size Comparison

 

The spec for the outer diameter will tell nothing about speedometer calibration. You need to look at the spec for the revolution per mile.

Different tire models with the same tire size will have different revolution per mile.

 

The calibration of the speedometer on my 2021 Prius Prime Limited with Dunlop Enasave 01 A/S tires is perfect—always 2 mph higher than the actual speed in the entire speed range—which means that they got the revolutions per mile exactly right within a few revolutions.

If the revolutions per mile differs more than 0.5% on the 17" vs. 19" tires, I would guess that their speedometers were calibrated differently. Chances are that they used the OEM-tire spec when they calibrated them, as tires from different manufactures can differ greatly on the revolutions per mile.