Odometer and Speedometer Accuracy

Every Toyota I’ve owned had a speedometer that read 2 mph high. I never swap wheels or tire sizes.

 

Agreed mine is the same. Indicated 37 MPH is actually 35 MPH on my Prius. It is the same for my BMW.

 

Speedometers read high, by design and legislation, to ensure you're never going faster than the displayed speed.

The speedometer more'n likely gets it's (accurate) info from the odometer, then adds a percentage when displaying. If you hook up to the car's OBD port, say with a ScanGauge II, you'll see speed value constantly lower than displayed.

 

I thought 2019 and newer PP LE did not come with onboard GPS. I don't think Toyota would use different methods of ODO reading based on the trim level.

 

That was a bad assumption. Built-in "customer expectation management" can alter the accuracy of the displays.

Get an OBDII-port engine monitor (I have an old ScanGauge-II, but many people use much newer, better, less expensive products) and compare the ECU internally reported speeds to the dashboard display and to your stopwatch or other references. It is expected that you'll find this OBDII-reported speed to be different than the dashboard display, and much closer to your stopwatch.

Recheck again after the tires wear down a bit. Just beware that tread depth changes don't lead to as much rolling circumference change as you'd expect from the simple circle equations. This is a separate topic.

As mentioned by earlier respondents, legislative pressures force speedometer displays to bias high, for speed enforcement purposes, removing the "but my speedometer said ..." excuse for most drivers. As best we have heard on the Prius, the display console simply adds a small increment to the number it gets from the internal CANBUS.

Separately, civil product liability pressures force the odometer to not read high. Very long ago, I received class action lawsuit notices for two of my cars (different brands) accusing them of warranty fraud because odometers were turning too fast. Neither of my cars suffered the alleged defect, both being fairly accurate, but the makers settled by giving 2% mileage extensions on their warranties. Subsequently, I have purchased a newer model of just one of those brands, and its odometer reads about 1.5% low. I don't recall Toyota being similarly sued (I didn't own one at the time), and have measured my Prius reading closer to just 0.2% low.

In summary, your car can use just a single speed & distance measuring system, yet still have odometer and speedometer reading different, intentionally, by design.

 

Both the odometer and the speedometer are derived from a mechanical connection to the transaxle.

They are however calibrated to different requirements.

In particular, the European requirements for speedometer accuracy are such that it is required that it never indicates less than the actual road speed. The US requirements I could find had a symmetrical error allowance of +/-5mph at 50MPH.

"The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).

0 ≤ (V1 – V2) ≤ 0,1 V2 + 4 km/h"

https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2010:120:0040:0048:ENDF

On analog speedometers you can often see the calibration limits marked on the dial as small extra marks;

There are practical limitations with GPS in that it is not available everywhere, eg underground, tunnels or in urban canyons.

kevin

 

When using highway milepost signs, check many subintervals and adjacent mileposts along the way. I've noticed numerous anomalies that must be accounted for in the calibration.

Some mileposts are offset from their correct positions, possibly for mounting convenience. Sometimes it seems they just wanted to make another post serve multiple duties, while other times bridgework makes for difficult mounting, so signs get moved off-bridge to an easier spot. These errors are non-accumulating, so matter only when an offending milepost was used as a start or end point.

There are also occasional discontinuities. These mostly happen when road segments are moved / realigned / shortened, but sometimes with no obvious changes (or the construction changes pre-date my personal history there). Some of these are marked with a special milepost of the form "NNN.XX AHEAD, NNN.YY BEHIND", but most are unmarked or unnoticed. Twisty winding mountain and river highways seem to have many more these than do open Interstates. I-90 in Eastern Washington has a single such discontinuity between the Columbia River and the state line, where about a half mile is "missing" at MP220 at Ritzville. I haven't been diligent enough to adequately check the western half of the highway. But considering the major grade separations across two valleys, each direction should be checked separately.

 

I severely doubt it - as mine hasn't got a GPS.

I just changed tyres - and my speedometer changed by ½km/hr against my TOMTOM GPS. Same size. But the original tyres, the GPS vs Speedo read slightly differently depending on pressure (I'd bump them up for higher speed driving). And between new and 60,000km, there was a slight difference too.

Tyre temperature? Road surface?

 

Correct. By design. However the ecu has the correct mph which can be displayed with a variety of obd2 connector tools. Here I am scanning the CAN bus accessed at the obd2 connector to provide accurate mph and engine coolant temp. About $40.

 

Where does it get the info for the "correct" MPH?

 

From the transaxle which has precise rotation measurements to facilitate the hybrid control. MG2's resolver.

 

I noticed the same behavior driving a Hyundai accent across the country. Odometer accurately matched highway markers and GPS. Speedometer consistently read higher. Car didn't have a built in GPS.

Seems to me like while both odometer and speedometer receive data from the same source (wheel rpm) the odometer displays the most accurate number possible while the speedometer displays a higher value...

Odometer pulling GPS data would be a bad idea. Bad weather, tall buildings, solar flares and alien radio signals amongst other things frequently disrupt GPS signal...

 

Yep - my (TOMTOM) GPS loses signal regularly. Generally when you most need it.

 

So it is in relation to the output speed of the transaxle - not the speed of the car, which could be affected by tyres, pressures and even tyre grip.

 

I guess it's finding the mean - without regard to any standard (or non-standard) deviation). As you go over bumps, the tyres would compress and release continually, never the same for any length of time, unless it's a very good road.

I recall seeing a video of the tyre of a car under full (savage) acceleration, then braking - and it flexed considerably.

Can't find it - but here's a Dragster wheel:

It's one of the side-effects of high-profile tyres - the tyre provides a deal of damping from road surfaces - as a low profile tyre doesn't.

How that relates - I'm not sure - maybe you were lucky? Interesting question though.

A wild-goose question - if you were to do the 369 miles on a track turning left continually - like NASCAR - I assume the differential effect of the transaxle would sort that out. Or not. But then, you'd be running different pressures left/right too. (I've read about extremely high on RH, crazy low on LH - but not sure if that's true.

 

It's more complicated. Suffice to say there are published revolutions-per-mile stat's for tires. TireRack for one has them. Any 195/65R15 tire (for example) will have roughly the same rev's-per-mile.

 

Conspiracy theories have moved from politics to odometers that work too well.