calculated vs. computer MPG - Please post your results

To design a good poll, we need to know the earliest manufacture date. We got our Prius in May, 05/09, so the manufacture date is probably 04/09 or possibly 03/09. I suspect our Japanese friends may have earlier ones.

Since a poll has only 25 responses and we have a distribution of indicated and actual mileage, I would use 5 buckets for responses break up the time ranges into 5 groups. The buckets for responses would be:

• < -2 standard deviation (STD) from median (0%)
• -1 STD from median (3%)
• median (5%)
• +1 STD from median (7%)
• > +2 STD from median (10%)

Using 5 time ranges, I would use:

• 02-03/09
• 04-05/09
• 06-07/09
• 08-09/09
• 10-11/09

The comes the instructions for how to calculate the percentage error. A table may help but this can blow up quickly. This is what I'd be after:

"Please calculate the percent of error by adding one or more tanks that you have both the indicated and actual mileage. Sum the indicated MPGs and sum the actual MPGs. Then take the difference of the two and divide it by the actual MPGs. More tank records is best but even one tank is enough."
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We would run the poll for 30 days.

Comments?

Bob Wilson

 

Filled up today, 635 miles on the trip, and calculated and computer mileages were exactly equal, at 52.3mpg, surprisingly. Prius III built 9/09.

 

I am beginning to believe that the more recently made prius's are more acurate than the early one. My last fill up the guage read 51.9 and actual was 48.14 MPG.

alfon

 

Last tank: MID - 55.0, Calculated 51.5

 

2nd tank, 375 mostly highway miles in San Diego (hilly) avg 45 mph

Computer = 54.9

Calculated = 52.0

mfg date = 9/09

 

I have compiled detailed data on my indicated mileage vs computed at the pump. I am showing an average 6.4% larger indicated value than measured. I can now pretty much subtract 3 mpgs from the indicated value to arrive at my probable real value. Does anyone know if an adjustment can be made to bring these values more in-line?

Mfg date 04/09..

 

comment deleted

 

I'm averaging a 6.7% difference (3.8mpg) between the MID and calculated over the last 5 fillups. Sadly, I'm also feeling the effects of the recent drop in temperatures and a few short trips:

Sept, dispay 60.8mpg, calc'd 57mpg
Today, display 54.6mpg, calc'd 50.9mpg

My car has a mfg date of 6/09.

 

For my first fillup:
Display: 51.6mpg
Calculated: 48.4

 

Build Date: 04/09
8th tank of gas after 4 months
MFD MPG = 48.4
Calc MPG = 45.6
Difference = 5.8%

After 6 months
MFD MPG = 43.9
Calc MPG = 41.7
Difference = 5.0%

 

Here is the Excel file that I have kept with the mileage calculations along with the trend.




View attachment Prius Mileage.xls

 

Cool photo.

 

338 miles / 7.75 gallons = 43.6 calculated. (I hesitate to call this actual, because it's just my second fillup and I'm not sure that I'm filling up these tanks evenly yet.)

Computer MPG = 46.4 mpg.

Calculated is about 6% below computer estimate, and I'm pretty sure that I'm not filling up the tank THAT unevenly.

Mfg. date of auto 09/09.

Tires: Goodyear Assurance FuelMax. press. 42/40 approx.

Sinclair gas, MAY have been E10.

Arkansas/Missouri, most driving on tank done in Ozarks, a hilly area, mostly between freezing and 50F, lots of rain.

To answer someone else's question, yes, I would go in to have this adjusted as part of my regular service, if Toyota would fix this for "free" (i.e., as part of the whole deal of owning the car).

 

Seems reasonable.
How did you figure the standard deviation? By taking what you have gotten in this thread? That would sound reasonable.

 

Love the spreadsheet. Do you mind if I use a copy of it for my cars?

 

manufacture date: 06/09; owned since 09/08; 4913 miles/ 94.2 gal = 52.155 mpg vs computed = 52.7 mpg

 

Just as a matter of interest, I was stationary at the traffic lights waiting for them to go green and the ICE was running to warm itself up. How does the fuel used during a stationary warm up get counted as far as FC is concerned?
Although very small, would the amount of fuel used account for some of the discrepency between displayed and calculated figures?

 

It is properly counted just like any other fuel consumed. The displayed mpg is calculated by taking all of the miles and all of the gas consumed since a given trip reset. There is no time variable involved and counting fuel has nothing to do with the wheels turning or not.

3PriusMike

 

Hey, Orf,

I don't think where you are you'll ever experience a cold winter like we get here in New England. When we get some severe weather such as snow & freezing rain that will combine & build up a thick coating of ice, it takes a while of running the heater & defroster (forcing the Prius ICE to run to provide the heat) to melt all of that junk off. One time last winter I had to run the car for about an hour to break all of the ice coating free & during that time, since the ICE (gas engine) was running but the car was NOT being driven to accumulate any miles, my MPG dropped about 10 MPG. So, any running of the gas engine without driving any distance will have a negative effect on your MPG (probably just not as severe as my example).

Ken (in Bolton,Ct)

 

I've tested several times, and I think the computer overstates mpg by about 5%.