2010 Toyota Prius FCD Inaccuracy

Can any of you guys check the accuracy of your speedometer? Mine is a full 3% off (on the fast side) which will only get worse as the tires wear. Funny thing is my scanguage is 100% accurate to my GPS... It really sounds like Toyota is trying something fishy here, this isn't rocket science. I'm sure my mileage is off by at least that amount as well.

 

Garmin shows 65mph and Prius shows 64
Local radar check shows 34 when prius shows 35.
Close enough for me

 

The speedometer offset is intentionally produced to meet the regulations.
The odo meter reading is completely different story from the speedometer.

Ken@Japan

 

Could you explain the regulations that require a speedometer to be intentional wrong?

 

http://www.unece.org/trans/main/wp29/wp29regs/r039r1e.pdf

Ken@Japan

 

Thanks, Ken.

I think the particular language of interest from the United Nations regulation is the following:

"5.3. 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"

 

Interesting, according to that formula if the actual speed is 70 MPH (for example) they think it would be OK for the speedometer to read from 70 to 79.5 MPH? do I have that right? If so it would be way to much tolerance for errors in one direction and none in the other. I don't understand the logic. Do they really think a speedometer that always reads fast is somehow magically going to keep me safe? Wouldn't it be more reasonable to simply say indicated speed should be equal to V2 + or - .05 V2. After all the chances that replacement tires are going to be larger or smaller are about equal.

 

I just did my second fill-up on my 2010 Prius II. The display said I was doing 62.5 mpg, I used exactly 5 gallons for 305.8 miles, so actual mileage was 61.2 mpg. Not much difference.

 

Just filled this tank.



Car took 12.075 gallons of gas. That works out to 66.28 MPG. The discrepency is 7.52 MPG which is 10.2%. That shouldn't be. My ScanGage was a lot closer for MPG.

View attachment 17253

And it had the gallons used almost dead on.



Wayne

 

once again the discrepancy. but still a very nice tank Wayne!!

 

I got this link from the main forum:
Fuel Sipper Smackdown 2: Which Car Gets the Best Fuel Economy?

There's an interesting quote in there:

Toyota definitely cannot ignore this. Hope there's a service thing that goes out in the future that addresses this issue.

 

So it looks like the odometer is correct, but the display is showing ~10% greater mileage than actual. Somehow the fuel side of the calculation is wrong. Wrong injector size factor? Or did they apply some other factor incorrectly?

This is a somewhat embarassing technical error. I hope a flash will fix it once they have tracked down the source. (My GenII's error is 0.97% after 27 fills, so I don't buy into various gas station conspiracy theories.)

 

gas station error was investigated and eliminated in my situation

 

Here's the "logic". You are looking at it from a totally different perspective than our government. It is not a safety issue at all but a financial one. A speedometer that reads slower than actual speed allows a driver to beat a speeding ticket. One that reads fast does not.

 

I haven't done a lot of MPG calculations, but I just took our 2010 Prius on an 84-mile trip and compared the odometer to a GPS unit and found that the indicated speed was about 1 MPH high at 55, and the roundtrip ~84 miles was within 0.5 miles of the GPS.

So if to the extent that the MPG is off, it is in the internal calculations. I only have a few fillups, and so far my by-fueling MPG is about 48 MPG, and my usual indicated MPG is a little over 50, so I'm not sure if mine is as far off as some. Could it be related to the model/package/options?

 

Add me to the list with discrepancies!
Still getting over 50 on the calculated but it's a big difference between 50 and 56-59!

 

Not only is the variation between indicated and calculated looking to be common, the interest in discussing it is growing.

I now count at least three threads dedicated to the topic. Perhaps we should look to consolidate these threads?

On the off-chance such consolidation does not occur, I'll post my latest results here.

My car has now surpased 6,300 miles, and I've filled it 14 times. True comparisons only could occur starting with the second fill, because I have no idea of the true quantity of gasoline in the tank when I purchased the car.

Here is a graph showing how the indicated (MID numbers) vary from the calculated (using fuel purchases). In the 2010, I've NEVER had a calculated fill show better fuel economy that what has been reported on the MID.

Here's a chart showing the percent by which the displayed (MID) mileage is actually higher than the calculated MPGs. It appears to me the difference is going to settle right anroud the 5% mark -- meaning the reported (MID) numbers are going to be about 5% higher than the number ahieved at through calculation of fuel economy.

 

I am getting between 1 and 3 (rounded off) gallon difference from computer indicated mileage and measured mileage (the computer is high). For reference, my indicated mileage will vary from 46 to 53 mpg. I believe that is a perfectly acceptable error rate. I use the Toyota recommended tire pressure on 15 inch wheels at this time.

 

I am probably completely off, but maybe (just maybe), the speedometer & FCD were calibrated for 16-17 inch wheels (which are standard in Europe)? So if you have 15 inch wheels on, it would naturally show higher speed & MPG. The difference would comply to the ~10% region, as:

15x3.14=47.1
16x3.14=50.24
17x3.14=53.38

so, for 16 inch wheels that yields
50.24/0.471=106.67, that is 6.67% error

and for 17 inch
53.38/0.471=113.33, that is 13.33% error

That would explain the difference... But its just my two cents. Has anyone tried out how accurate the speedo readings are?

EDIT: Sorry, I did not read the whole thread. If speedo & odometer readings are correct, my hypothesis is obviously not valid.

 

Last fill-up. Indicated 51 MPG. Actual 48 MPG. I've had to stop telling people I get 50 MPG.