Speedometer Accuracy

Speedometers intentionally read high.
Probably for legal liability reasons.
Multiple discussions exist on this issue.

 

Yes, it's been noticed before. To my knowledge it cannot be adjusted. Just something you have to live with in these cars.

 

You mean like these?
Prime:

Speedometer off??? | PriusChat
Speedometer/odometer reads high ‘18 Prime | PriusChat

Gen4:

Speedometer incorrect and Cruise Control Wrong | PriusChat

 

This may not be a good assumption. Odometers are usually quite accurate and independent from the speedometer calibration. Since you are already using GPS to check speed, check your odometer as well.

 

Speedometer lies, odometer tells the truth. The car knows the actual speed internally, but it reads high on the dash. It even knows the accurate speeds of each individual wheel for things like vehicle stability control. More details here: Your Speedometer Is Wrong - Speed Calibration Inaccuracy In German, American, And Japanese Cars - Thrillist

 

fwiw, a Gen 3 has the same "problem". It's not something I'm concerned about. I don't drive slower than anyone else and I don't drive faster either. In traffic, I follow it, when I'm alone on the road, I go what I feel is a safe amount of speed over the speed limit to neither get caught for speeding, nor get annoyed by going so slow when i'm the only one on the road.

 

^Oh?

I'm curious about this, since my presumption was that the odometer used the same input as the speedometer - from the VSS sensor.
Why would they be different and where does the odometer get this different input from?
Priuses have always been inaccurate as far as their efficiency self-reporting is concerned AND coincidentally (wink-wink!) they've always overestimated their fuel efficiency for some "strange" reason. One presumes that speedometers will vary by a percentage point or two as you slowly grind off about 8/32" of tread from your tires, and usually tires are pretty central in determining speed over ground.

Since passenger cars are usually not equipped with an inertial navigation system, and I'm "presuming" that they do not use satellite navigation to suss out the differences between speed through the air versus speed over ground, I'd be interested to see how the vehicle differentiates speed and accumulated distance(s) traveled, and what specific sensors they use for each.

Of course, the next time you take a long trip with your car you can always go 'old school.' and set your trip odometer (or write down your main odometer reading) and use highway mile-markers to gauge your inaccuracies.
DOT workers who plant those mile markers may not use differential GPS to put the sign in the EXACT spot, but one presumes that over...say....100 miles that they're going to be close enough for you to see if your VSS/odometer/speedometer is off by more than 2-3 percent.

If you want repeatability?
Do it again on your way home.


Speedometer Scandal!

 

AFAIK, the odometer and tripmeter both were dead on accurate when comparing to the mile markers along the interstates. Speedometer maybe off 2-3 mph at higher speed, but at lower speed it always shows the exactly the number shown on one of the roadside radar displaying your speed in 30-40 mph zones around my town.

 

Exactly how hard would it be for the software that counts wheel revolutions to add 2 mph to the speed indicator and display an accurate odometer reading?
I’d estimate about zero difficulty.

 

If you set tire size / circumference in the software, yes. That is not a normal option for us though,

 

Here's a link that discusses the "problem":

Your Speedometer Is Wrong - Speed Calibration Inaccuracy In German, American, And Japanese Cars - Thrillist

Anyway, it's a margin of error basically for tire differences and wear.

 

Yes.

Almost every new vehicle produced recently comes out that way......mostly on purpose, the be sure that it doesn't read LOW.
It has been discussed here before at length.

It is not REALLY a problem if you just learn to live with it.

 

The odometer does get the same input, but then the speedometer software displays it with an offset. Intentionally, as this meets requirements in various jurisdictions. I have compared my odo to a GPS on a long trip, and they are quite close (a mile or so at a thousand).

All of my recent cars (since 2006) have an optimistic speedometer. 2006 Miata 2 mph off at 60, corrected when I upsized the tires. VW TDI off by 2.5 mph, Prius off by 3 mph, Subaru Outback off by 1 mph.

It's an annoyance rather than a problem. If I want to actually go 60 mph, I set the cruise at 63 and check my GPS for an accurate over-ground speed.

In the later 2000 decade models, Honda had an odometer error that resulted in a class action lawsuit and extension of the 36K warranty to something closer to 39K (I don't recall the numbers). Automakers will be as accurate as possible in odometers, but have no compelling reason to make speedometers accurate (and have legal reasons to make them optimistic).

 

Makes sense, but then this means that the folks in Aichi might not be as honest as I've been giving them credit for....and YES. that's sarcasm!

If the odometer is accurate to within 0.01% then the fuel efficiency measurements should be as well.

If they're not, in a car that prides itself in fuel efficiency, then that tells me much......

 

Subaru too. Though neither my pre-lawsuit Subaru, nor Honda, displayed the alleged error. Both companies settled for approximately 2% warranty extensions, plus paying for some slightly out-of-earlier-warranty repairs. Plus enough to some class-action product liability lawyers to keep them wealthy.

My post-lawsuit Subaru has an odometer that reads roughly 1.8% low. But its MPG display is pretty much right on.

I don't remember Toyota getting hit with any similar odometer-warranty-fraud suit, but then I didn't own one at the time. My Prius odometer reads about 0.2-0.3% low.

 

When I had a scangauge plugged into the OBD port, it would display speed a little lower than speedo. This would I think confirm, that the car knows the more accurate speed, internally.

 

Then compare that OBD port speed reading to a GPS unit. I think mine matched when I checked long ago.

Previous discussion indicated that on a Prius at least, the speed offset 'error' is added right at the dashboard display.

 

I wonder if the speedometer is intentionally set too high for fuel economy reasons? For example if the EPA test cycle says something like drive 65mph for 10 minutes, there will be a slight savings if 65 on the speedometer means the car is actually going 63.