Prius C MPG Calculation Method Comparison

Fascinating, thanks for sharing! That seems to be spot-on compared to the observations I've made with my calculated vs. OBD indicated. It seems to average about 2 MPG optimistic versus reality.

Props for putting it into a spreadsheet

 

Boo! I was wanting to get better numbers as is, now I know they are in actuality even worse! (Guess I should have suspected from my fuelly score). I feel embarrassed being below 50!

 

Wow you put a lot of work into tracking everything. That is a really nice graph too. Do you have dates on the fill ups?

 

Maybe the calculated value doesn't account for ethanol-contaminated gasoline.

 

My data is presented in your format below for comparison. I suspect my calculated values are being influenced by smaller tank fills and a high variance in the performance of the auto-shutoff feature on the fuel pump nozzles. As I've whined about elsehwere, due to local practices of fuel companies messing around with prices I'm forced to buy petrol at the intervals they define, not when I need it.

Also note that my vehicle reports consumption in l/100km to 1 decimal place which I've converted to MPG here, which in turn causes it to jump around a bit.



I've also done a version in my native units of measurement partly just to indicate that by my standards, MPG is a bit of a finnicky measure. My car is mostly within one position (a difference of 0.1 l/100km) and sometimes there is a bit of bounce that I attribute to the pump differences, which balance out typically on the next fill up.

 

I don't always record indicated MPG but after checking a few tanks, I've seen an average of about +2.5 MPG indicated vs. calculated.

 

Rob and Oldwolf,

Congratulations for the effort you have put in to compile, plot and share these statistics. I see that Rob's indicated fuel consumption matches well with hand calculations, but Oldwolf's indications are consistently on the optimistic side compared to hand calculations, by a factor of about 5 %, which is quite a lot!

I will start a similar exercise on my side to see what I am getting.

I personally doubt that Toyota would intentionally bias the calculation (and therefore the indication) towards an optimistic value. I believe that the lack of consistency that we are seeing between these two sets of data could be attributed to one or a combination of the following factors, which are listed in decreasing order of probability:

- errors in measured fuel consumption (due to system tolerances)
- errors in measured speed ( notice a difference between GPS speed and indicated speed?)
- errors in fuel quantity readings at the gas station pump (unlikely because, at least here in Canada, gas pumps are strictly regulated and they must be calibrated at regular intervals)

Will get back to you with my own results later.

Thanks again.

 

Really? After looking at the OP's graph, LOL? And considering that fudging the numbers will make a lot of owners happy, and get Toyota free word-of-mouth advertising, albeit false?

At least the fudge factor seems to be reduced, with the C, and maybe with newer models in general. I've been calculating our 2010 reg Prius' mileage for close to two years now, and have an average error of 8.06%. Always in favour of the house.

 

I just did this comparison using Trip B reset at the pump, ran a whole tank (down to a couple of pips), then filled up at the same gas station, exact same gas pump. I even made sure the car was parked in the same relative spot so the tank was not out of level.

Results were 52.4 MPG indicated on Trip B to 50.9 MPG by manual calculation. <3% Error.

This was using Real Gas (a.k.a. "Pure Gas" a.k.a. "Ethanol Free") for other scientific purposes Just under 2k miles at this point. Daily temperatures between 50F during the day and 25F at night. Mild grill blocking.

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Maybe there is a standard leakoff/evaporation rate of some % per tank? That would theoretically yield a lower error the faster you used the gas. Possibly the fuel pumps are calibrated to within a tolerance band of +/- some % and somehow they are all magically calibrated closer to the more profitable side of that tolerance band... something only a Prius owner would ever notice. There would also be an effect from the contraction of the gas coming from nice warm underground tanks and going into your car's tank (significantly higher than water for the same temperature change). I'm just saying there are a couple more sources we should investigate for where this error is coming from. ​

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From Wikipedia:​

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In the United States, the National Institute of Standards and Technology (NIST) specifies the accuracy of the measurements in Handbook 44. Table 3.30 specifies the accuracy at 0.3% meaning that a 10-US-gallon (37.9 L; 8.3 imp gal) purchase could vary between 9.97 US gal (37.7 L; 8.3 imp gal) and 10.03 US gal (38.0 L; 8.4 imp gal) as to the actual amounts at the delivery temperature of the gasoline.​

The reference temperature for gasoline volume measurement is either 60°F or 15°C. Ten gallons of gasoline at that temperature expands to about 10.15 US gal (38.4 L; 8.5 imp gal) at 85 °F (29 °C) and contracts to about 9.83 US gal (37.2 L; 8.2 imp gal) at 30 °F (−1 °C). Each of the three volumes represents the same theoretical amount of energy. In one sense, ten gallons of gasoline purchased at 30°F is about 3.2% more potential energy than ten gallons purchased at 85°F.​

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With two years of data your margin of error must be small but I don't know if you can claim three digits of accuracy. Probably better so just say "8%." With the Prius C the mpg difference just might be inside the margin of error. I don't know yet.

I use the car's mpg meter as a relative number. It lets me know in relative terms if I'm doing better or worse than I did the day before. For example I tried an experiment where I drove very aggressively and stomped the gas and the brakes. The MPG meter said I lost about 15%. That is what I wanted to know.

 

My thought is that the engineer that designs a computer that shows the driver is getting less than actual fuel mileage would be taken out and flogged. We have seen too many instances of this for me to believe otherwise. And not just Toyotas.

 

Hi Oldwolf,

Yes, I 'm seeing the same trend. I've converted to MPG for comparison:


My per tank avg error is -3.3 MPG and STD is 2.3 MPG. Most errors are between 2.6 and 3.0 except 3rd fillup at 4.2 and 7th fillup at 7.7 which were refueled after long road trips.

I also track Lifetime FE which is more consistent:


My Lifetime avg error is -2.6 MPG and STD is 1.3 MPG.

Vincent