Odometer and Speedometer Accuracy

I once tried "tire radius" stuff, and couldn't get things to reconcile. Then realized that the contact patch on the ground is not a circle at all, but instead a flat patch. The circle equations just don't work for this as one might initially expect.

Then learned that tires have RPM (revolutions per mile) specs, which have more to do with the effective rolling circumference of a squishy object. Some arithmetic revealed that this RPM spec corresponds to a radius point somewhere underneath the actual tread. Inside the tire.

Don't focus solely on this tire radius stuff, it is a rabbit hole. It is more complicated, look for other approaches.

 

If I changed my v from stock 205/60R-16 to 215/60R-16s the tire circumference would go up and the revs/mile would go down by 1.9%.

My actual speed and odo would be 1.9% higher but the displayed values would be the same as with the factory size. However I still would not be speeding if the limit equaled my displayed speed because of the ~ 3% higher mph already built in. The error would be reduced by 1.9%. I would end up with an overstated odo because of the increased tire circumference.

Interestingly, the v's master cylinder's skid control assembly (brake booster) is a different part number for 16" vs 17" wheels. I am sure other devices on the car are adjusted for stock tire sizes as well.

If I changed my stock 16s to the factory low profile 215/50R17's, the displayed values would stay the same but I would lose 0.8% on the odo and actual mph. In this case the factory overstated mph would increase by 0.8%.

Moral of the story, stay with stock size for odo accuracy. But Toyota's decision to overstate displayed mph keeps you legal.

 

I've run Waze in Android Auto or my phone for a long time now.... I've got the GPS based MPH turned on in it, and run it pretty much all times I'm in the car. My 2013 Prius C, and now my 2021 Prime consistently report roughly 3mph OVER on the display what GPS based speed does. Other cars I've owned in the last few years it's always pretty much dead on. I also recently took a long drive following my brother in his new VW, he had his cruise set at 75MPH, while my Prime with the auto-adjusting cruise would ride behind him reporting we were doing 78 the entire time (and Waze/GPS telling me we were doing 75). My 2cents on the matter, probably owe you change....

 

My first experience with mpg display was with an 06 Honda Civic Hybrid, and it was spot-on, or slightly pessimistic (under reported mpg). With our 2010, Toyota's basically lying; with over 10 years of records the error's currently at 7.2%.

 

Yes, MPG display accuracies remain unregulated, and a high bias appears to be the industry norm. The amount of bias you are finding seems consistent with the Prius family from 2010 on. Some older hybrids were much more accurate, Mendel points out one example. I also seem to remember that some car reviewers testing prototypes found their displays to be fairly accurate, these significant biases didn't appear until forum readers were able to buy actual production models.

 

Gas pumps only have to be accurate to 3%.

 

Could have been from me: I read Wayne Gerdes's (cleanmpg founder) 4th gen Prius review, and he found the mpg display accurate. Only hitch: it was a pre-production model, slated to be crushed in about a week.

 

References?

My state's weights and measures webpages are very opaque on this at the moment, hiding the actual standards, but I'm finding news stories of pumps failing inspection for errors of 1.2% and 0.4%.

 

Wayne was the specific reviewer name coming to mind, but I thought this issue happened on more than one vehicle he reviewed, and also on other reviewers.

 

I don't know - I read it in my state's regulations a long time ago. Maybe it's better now but that's what it was when I read it.

 

I just found a couple very old items referring to some states having a limit of 6 cubic inches of error in 5 gallons, which translates to 0.52%:
Inspection Reveals Variations in Gas-Pump Accuracy : NPR
https://www.mcall.com/news/all-watchdog-080308-gs-story.html

A much more recent Australian industry article mentions a 0.3% limit:
https://acapmag.com.au/2019/03/are-your-pumps-accurate-nmi-inspectors-visiting-sites-in-the-wake-of-startling-increase-in-pump-inaccuracy/

 

This wiki article on gas pumps says 0.3% too, presumably USA specific:

https://en.wikipedia.org/wiki/Gasoline_pump

It's some ways down in the article, you could search for this string:

The standard accuracy is 0.3%, meaning that a 10-US-gallon

 

Even if the gas pump is accurate to 0.3%, there still are quite variables in the amount of "full tank" from one fill-up to the next, I would think. I try to be consistent, but depending on where I fill up when the auto-shutoff stops the delivery there seems to be a difference from one fill to the next even at the same pump.

 

In my 2021 Prius Prime Limited with Dunlop Enasave 01 A/S tires, the speedometer shows exactly 2 mph higher than the actual speed at all speeds.

 

i always drive 32 in a 30mph zone