Calibration of 2010 Prius MPG indicator
 

With 109 samples, manually collected from the PriusChat community, we now have enough data:
http://hiwaay.net/%7Ebzwilson/prius/pri_2010_630c.jpg

The blue 'dashed' line is the desired, calibrated values. If the indicated mileage were accurate, the data points would bound either side of the dashed blue line. As for the upper, outliers, they are still sparse but they seem to be around the trendline. This is not enough to 'bet the farm' but enough to not worry too much if extended.

The other interesting aspect is the double-humped, MPG distribution:
http://hiwaay.net/%7Ebzwilson/prius/pri_2010_610a.jpg

This closely agrees with the double-humped distribution from the small number of user mileage reports from the EPA database:
http://hiwaay.net/%7Ebzwilson/prius/pri_2010_610.jpg

At this point, the ball is in Toyota's court. Absent some way to modify the software that calculates the indicated MPG, we just need to understand the problem and depending upon the requirements, there are these options:

• use pump MPG for accuracy and indicated for relative MPG performance
• use correction formula: actual = (.85 * indicated) + 5.3
• use 3d party MPG indicator: scangauge, supermid or others (mpguino)

Feel free to share this with other Prius groups and thank the PriusChat community for their data contributions.

Bob Wilson

FYI, Yoshi built the supermid specifically for the prius, and he and the folks testing it had the fuel consumption dialed in to within 1%, he also made some key contributions to the mpguino.

Hopefully Toyota will listen and correct this issue. I'd be horribly disappointed if my MFD was as far off as it is.

...does the Prius Owners Manual 'fine print' read: "...your milage may vary"?

The NHW11/NHW20 had the bladder tank so no one could really check the MPG display without special instruments. If you had special instruments, you didn't really care because you'd use the more accurate source. Regardless, we're letting Toyota know about the problem. In fact, I'll open a case too, now that we know the characteristics of the problem.

One good thing is with the extensive software controlled systems, I suspect Toyota can easily fix this with a download. Considering we've only had our 2010 Prius for 90 days, I think this is remarkable progress.

Bob Wilson

It looks like what may have happened is that the indicator was calibrated for ethanol-free gasoline (which has more energy per unit volume than ethanol gasoline). Hence the few points that were on or above the indicated line.

That is an interesting hypothesis.

Ken@Japan reports a Japanese engineer told him they designed it to read 5% high. I suspect Toyota may rethink that approach someday. Regardless the formula derived from the statistical data seems to be more accurate than a straight 5% bias.

Bob Wilson

After the first 4 tanks on my Prius I am finding that it registers almost exactly 5% high on every tank so far. One on 10% ethanol (10% worse gas mileage) and the other 3 on real gas. I had a J.C. Penny fuel mileage indicator in 1975 that allowed me to calibrate it by putting in actual gallons at the pump. Surprised that Toyota hasn't been able to replicate a 35 year old technology?

E10 doesn't have a 10% drop in mileage. E85 might have a 10% drop in mileage; but not E10.

I agree, though about calibrating the stock FE indicator -- the ScanGuage II does it, and so could the factory unit.

Mine do Every time. Three cars, all do the same. Many tanks... Same result.