(Scientific) study on the calculated vs computer fuel economy discrepancy

Interesting - keep us all updated.

Is correct as simple as a software update or does the whole sensor need replacing to correct this for future?

Is it in Toyota's interest to correct this?

 

I'd vote yes.

 

A lot of other vehicle manufactures are equally optimistic so maybe there is a general method for setting such systems up and calibrating them.

 

I've only had a chance to glance at your data, but other factors to consider are odometer error and differences introduced as tires wear out.

See Speedometer Scandal! - Feature - Auto Reviews - Car and Driver (there's also a PDF to download entitled Speedometer Accuracy) and Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires. I posted the C&D article before at http://priuschat.com/forums/gen-ii-...oting/28808-odometer-accuracy.html#post368671.

 

Honda, at least for the second generation Civic Hybrid, starting in model year 2006, is slightly pessimistic. In my experience in-dash mpg reported turned out to be about 98% of calculated mpg. It can be done.

 

I agree with F8L on this one, in general (there can be exceptions) the automakers will side on making the displayed MPG optimistic. Many people wouldn't even know the difference because they don't bother to calculate their MPG. Having the displayed value = truth in their eyes, so why not make the truth a little more "rosy". The displayed value is usually close to 95% accurate, so for most people, this is "good enough".

However, this board is populated by scientists, engineers, and people who like accurate AND precise data, thus tend to be bothered by a determinate error in the displayed MPG. For my car, the average determinate error is pretty constant over a number of fill ups, so I can adjust the displayed value to the real value with arithmetic.

My brother's last Mazda also overestimated his MPG as well, about 5 - 7% I think (typically about 1 to 1.5 MPG on a car that got about 24-25 mpg). While 2 points doesn't make a reliable data set, the issue isn't exclusive to the Prius or Toyota. It is more evident in the Prius because a 5% difference on a 50 MPG car seems more significant than a 5% difference on a 20 MPG car.

 

Is the error rate similar for all 3 different methods of measurement? ie US mpg, UK mpg and litres/100km? Or are some methods more accurate than others? Is something lost or gained in the conversion?

 

I have kept records of all of my tank fillups for two years now and my average error is 6.3%, right in line with SCote, and in US mpg.

 

what are y'all basing the difference on, is it the amount you fill your tank up with? Unless there is a way to measure the exact amount you have in the tank there is no way to accurately measure the mpg (mileage/gallons), even a few tenths change the mpg and hence accuracy of the computation. just a thought

 

There is another conclusion that may strike a chord with the conspiracy theorists among us. All gas pumps are off in favor of big oil. They are charging us for more gallons than we actually pump. Discuss amongst yourselves, LOL.

 

The error will be identical, regardless of the method of reporting mileage, units.

Ours average error is currently at 6.4%. Greatest error was 9.6%, least was 3.8%.

(There was a couple of anomolies I threw out: my wife tanked up at a station that was only taking cash, didn't fill completely, then I topped it up, long story).

I'd like to be a fly on the wall when Toyota was setting up the programming:

"So how much fudging the numbers do you think we can get away with? What's the sweet spot? High enough to push up our high mpg rep, but not so high that the BS starts to trickle out..."

 

This is interesting data and the error seems higher than I would expect. Typically I see reports of 5% error for the MFD higher than calculated . Both of the apparent causes of error COULD be corrected with software. As stated there is an ambient temp sensor, so temperature variations could be included and if the flow rate is wrong, that could also be corrected. The flow correction COULD even be setup for the user to "adjust" after a few fill-ups.

The method of calculation is irrelevant to the effect MPG, KPL, or L/100K. All of those are know mathematical constants for the conversion and the unit should be based on the user's display and pump combination.

All US states have a Bureau of Standards which verify the accuracy of the pump. The pumped volume is accurate to 0.5%. Now WHEN the pump stops filling the tank will vary from pump to pump, but any low fill one time will be washed out by a high fill later.

As stated earlier, most consumers only care about what the display says. They don't bother to (or do not know how to) calculate their MPG at each fill-up. So, it is in the car maker's interest to "fudge" the numbers on the high side and keep the consumer happy (as long as it isn't "too far" off.

All in all... very good data and analysis!

 

That may be true with most cars that display MPG's. However
Prius owners are of a different breed, and most do calculate their mpg's. Just see the response from this forum....

alfon

 

Do you want to collect data from others? I've got an excel spreadsheet set up that has my fuel mileage data from April 2010 to date. It has the miles per tank, fuel added, displayed HSI in mpg, and calculated mpg. I also have a couple of columns that calculates how much gas I would have put in were the HSI correct. I did this so I could see the difference in average calculated MPG vs average HSI reported MPG.

As a bonus, I just added some columns calculating HSI displayed vs Calc average. The difference does seem to increase in the winter time and decrease in the summer time. Overall difference after 1 year was 5.9%. Current difference (lifetime) is 5.7%. (62.5 mpg HSI displayed vs 59.1 mpg calculated)

I can email it to you (if given an email address).

 

Oh yes it does, especially the last two years. We have had record heat of days over 100+F and record cold last winter of days in the 20F range.

My 6.3% is the average over these periods. It moves up and down from fillup to fillup. This average smooths out all of the tank fillup differences, temperature differences, etc. I keep all of this info in a spreadsheet started when I bought the car in August 2009.

 

It will take awhile to compile the temperature differences, as I'd have to look up 18 months worth of temperatures, and there will be noise because sometimes I fill up in the morning (on my way to work) or at night (coming home from work).

As far as filling up, I tend to fill up to one click past the initial click. I do this in case the first filling didn't get a good seal and shut of prematurely (rather than to see if I can fill the damn thing to the neck...gurgle gurgle gurgle). I probably don't need to do this practice, because in the 18 months I've had the car, I haven't run into this issue. Typically, the second click will occur after adding and additional 0.2 and 0.3 gallons of gasoline. That has been consistent.

If you think this data will have too much noise, let me know and I'll just start collecting data going forward. If you think there is still value, I'll back calculate the avg daily temps.