Here's Why Prius Drivers Drive Like That

Thanks for posting. Interesting article.

My Prius averages around 6% optimistic MPG reading and speedo 1 mph lower than actual everytime I drive past those radar speed signs.
Not sure how the speedo would effect the MPG reading on a car.

 

1) They are required by LAW to not show a number below the actual number, and given variations in tires, tread wear, etc. they err on the side of caution.

2) Every single other car made for the US does EXACTLY the same thing. It has NOTHING to do with Prius.

 

Something else to add, logically the next question people will ask is since the MPH is showing more does this mean the car is racking up more miles more quickly than it should?

I actually looked into this and the answer is no the mileage logged is still accurate. You can see how I was very concerned because going 90 mph the Ferrari is off by 6+ mph... Obviously I was very much worried about racking up the miles on that car....

 

There is no corrolation between speedometer inaccuracy and the the odometer reading. If the speedometer is reading high, it doesn't follow that the odometer display will also high: there are bucks riding on odometer accuracy, for warranty claims, and so on. If the odometer (with the stock tires) is reading falsely high, there would be lawsuits.

Case in point is Honda: there was a court case when odometer inaccuracy did occur.

 

There certainly was with mechanical odometers and speedometers. As for the electronic versions on Prius, I suppose they could have completely different sensors for miles and speed but in that case, there the speedometer should use the same sensor and be 100% accurate if the odometer is 100% accurate.

 

That may be so but there are many, many folks on here that have confirmed that the on-board MPG computer (not the odometer) is about 5% optimistic compared to hand calculation.

Not sure how "connected" the actual odometer and the average mpg display number are but that is the experience many of us have documented.

 

Yes, my calculated mpg comes in 8% lower. But what I'm saying (thought I was clear): there's no correlation between Toyota's BS mpg meter and it's odometer. Or there better not be.

 

I've tracked mileage with GPS and with the OE 195/65/15 tires at 40psi the odometer was very accurate despite the speedo being approx. 1.5mph too fast. With the 205/50/17 tires the speedo was dead on but the odometer was reading lower than actual.

You can also use an OBDII interface like Scangauge or Torque and see the ECU is reading 2mph lower tan what the speedometer is reading.

When in doubt, go measure. Don't make guesses.

 

I note that this regulation was updated in 2010, but I would say that it is functionally obsolete since ABS has become a standard feature and accurate, ABS pulse wheel, 4-wheel averages are now used to determine vehicle speed. I would say that my Prius reads only 2-3 mph high at 65mph while my Audi and Volvo are each within 1 mph (but not necessarily reading high?). All per the same Magellan GPS. Legacy magnetic speedometers were rather inaccurate and needed the range of ECE-R 39. Tire wear (new to worn) accounts for about a 2% variation.

 

But ScanGauge is using the same Prius sensors that create the lack of accuracy in the first place and the GPS's have their own error range compounded over miles traveled.

More a case of which measure do you choose to believe. Interestingly the only "legal" measure is that of the odometer which, based in most cases on speedometer reading, is allowed a 10% accuracy by law.

I would trust the radar instantaneous speed readouts especially when averaged over several units which I pass daily. With the radar speed matching the car's speedometer, I would then trust the odometer. In my car's case, that seems to be with the 205 tires.

 

Could it be that the sensors are accurate and that Toyota programmed the speedo to read fast to reduce liability risk?

Actually, I trust my GPS devices more than the speed radar. If I am tracking at better than 12ft. accuracy I am confident in the GPS readings. The fact that the GPS and Scangauge readings are nearly always the same (independently) means more to me than the occasional speed reader I may happen upon.

I play with these monitoring devices so often it feels like a part of my daily routine. With so much observation you cannot help but notice trends.

Remember, with the 195/65/15 (25") tires the odometer was pretty damn accurate. With the 205/50/17 (25.1") tires the odometer was reading too slow. Over the course of 400 miles the odometer would read as much as 7 miles lower than it should have. My odometer error was between 1.69%-1.8% when using the Ecopia EP422 205/50/17.

Source: Bridgestone Ecopia EP422 Review - 17" Tire | PriusChat

 

Actually it's pretty handy. It means I can drive with the speedo showing the speed limit and rarely hit the brake downhill.

By law the speedometer cannot show a lower speed than the actual speed. So the car has to handle anything that would cause variation and the easiest way is to use a large margin of error.

 

Like tire model differences (revs/mile), tread depth, inflation pressure etc..

 

There's no liability from accurate gauges. Conversely there is liability from inaccurate gauges though the auto lobby has garnered considerable protection for itself on the issue with the +/-10% accuracy rule.

Prius is inaccurate entirely in its favor, exaggerating speed and mpg. Bad behavior.

I'm glad my 205 wheels were able to fix Toyota's false readings.

In the case of Prius drivers perceptions by other drivers (this topic), Prius's bad behavior makes Toyota look good and makes Prius drivers look bad, driving slower than they intend.

 

I enjoyed the article as well... thanks OP for sharing.

Amazing what a few swigs of the Kool Aid can do... he must have dove head first into the vat if he's replacing his Mustang with a Prius c...

 

I rely on the MPH display from the ScanGauge II. It always reads lower than the dashboard display. The average MPG of the SGII is usually pretty close to the Prius value. Both are usually higher than my calculated MPG.

 

The discussion about a disconnect between the speedometer and the odometer caught my attention. Reverse engineering that, I wonder how you would go about it.

If I were setting up a speedometer, I would calibrate it for the largest diameter such that when the tire decreased in diameter, the reported speed would go up with age.

Not for sure how you would make the odometer more accurate than the speedometer reading.

The display being off is an unrelated issue. It's obvious that Toyota programmed the estimated mpg to be around 5% optimistic. Someone let the marketing guys in on a meeting.

 

Too bad they don't use an sonar detector that could get the MPH by bouncing a signal off of the road. That would be the most accurate. Or at least they could use that to correct the speedo based on wheel RPM.

 

Just what we need, more things to give me false radar detector signals!

 

I compared my speedo to two other satellite devices today and it's showing as being out by very nearly 10%
To me 10% seems a little over the top. Does anyone know if the dealer can adjust this?