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Computed fuel economy vs. calculated fuel economy

As you know if you drive a Prius or have been following this group, it is difficult to calculate fuel economy accurately with the Prius because of the bladder in the fuel tank that affects the ability to fill the tank the same on successive fill-ups. The only way to get a reasonably accurate calculation is to keep track over a number of fill-ups. Since I got my second Prius, I have kept an Excel spread sheet that calculates the average fuel economy per the computer and the actual fuel economy. In the first 955 miles since the car was brand new, the average fuel economy per the computer is 49.717 mpg. The cumulative fuel economy per my calculations is 49.454 mpg. Now, I realize that

955 miles is not very far, but so far I am truly impressed with how accurate the computer is.
Reply to
dayoff53
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I've been doing the same thing for over 6,500 miles now, and my numbers are 46.89 vs 46.28. After three rather revealing tanks where it was clear that I didn't fill up all the way, I've come to trust exclusively the MFDs numbers.

John1701 from most of the Prius forums challenged my conclusion that the computer was just as accurate and suggested I look at his 120k+ miles of data. I pulled his data into my spreadsheets and found that the numbers were within 2mpg of each other, too!

I track both, but I trust the MFD.

Reply to
Miwaku

Miwaku wrote: After three rather revealing tanks where it was

After receiving my car with a full tank, my first fill-up clearly did not fill the tank - resulting in a calculated economy of over 63 mpg. The second top-off before a long trip, likewise. Finally, the third fill-up seemed to actually fill the tank and the average came down to the numbers I gave at the beginning of this thread. I will be filling up again Friday morning - back at the same station where I had to fill-ups that didn't. I am interested to see what happens this time.

Dave

Reply to
dayoff53

I'm in the Boston area. I bought a 2006 Prius on 7-8-06. I've been comparing computer vs acutual at each fill up. To date there is only a

2 mile variance and an actual milage of 49 mpg.

I traded > As you know if you drive a Prius or have been following this group, it

Reply to
sthink

If you stop pumping gas once it clicks off you will get an accurate MFD reading. I paid good money for my MFD not to operate correctly. I only wish I had calculated MPG each time I filled up the tank before I had a Prius.

Reply to
indydoug

I have a 2004 Prius that I bought used in 2005 after it have been in a accident. Seemed that when they reset the AC Compressor they put to much

134 Feon in it and caused the engine/motor to run strange. The Insurance company then ended up paying of the vehicle for the original owner and I was able to get the car for about half the price. The Dealer Ship in my area found the problem in minutes and that was all that was wrong with it. Having only about 7K miles at the time it seemed like new.

As for the mileage, around town with stop and go, with top speeds of

35-50mph. I usually get 50-54miles per gallon. On the high at 50-60 cruising it's more about 50miles per gallon. Finally at 70-75mph it more near 45 miles per gallon. I currently have about 30K on the vehicle, and as some of the others, keep track of the gallons both as indicated on the readout and actual figured from the consumption of fuel. I also use a GPS to verify the mileage. Which overall is fairly close on the mileage, but some times off on the speed by usually the car indicating slightly faster by 1-1.5mph.

It may be a subject of discussion, however I found that accelerating at a more moderate speed then just slowly getting up to speed often gets better results. I believe this has the Motor helping out more then the engine. Seems like when the engine is running with out the motor drive the display stays in the 20mpg. However at the moderated pace it also stays in the

20mpg, a little lower maybe. But I'm up to speed quicker and back at the plus 50mpg.

Really enjoy reading the comment of this group.

Reply to
Donald Gibbs

To follow up: I have now filled up my Prius seven times and driven 2722 miles. My calculated fuel economy since the car was purchased (with 1 mile on the odometer) is 51.212 mpg and the compuer average for the same period is

51.674 mpg. With less than .5 mpg difference between the computer and my calculations, and given that the odometer only registers in whole miles (I don't use a trip odo for this) and the computer only shows economy in tenths of a mile per gallon, this seems remarkably accurate to me. On any given fill-up the computer may differ from my calculations by as much as 2 mpg, but I attribute that to differences in fill levels, the bladder, and other mechanical variances. If you would like to see my spreadsheet, it is available here:
formatting link
Thisis my first use of Google's new online spreadsheet. Some formattingdidn't transfer exactly correctly from Excel, which is why you sill seesome cell borders that don't look right. Feel free to steal myspreadsheet and adapt it for your own use. Dave
Reply to
dayoff53

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