calculated vs. computer MPG - Please post your results

2013 Persona - First fill up 452 miles 8.9 gal (50.8 mpg)
Second fillup 261 miles 8.1 gal ( 44.5 mpg)

what gives ?

second set of driving, I did encounter some some traffic jams on freeway but still numbers are way too off. On each individual trips it did show 50+ usually with one or two less than 50 mpg. But combined all together, I am disappointed.

 

You got an error on your second fill-up, you probably meant 361 miles.

There is a thread here that will probably help you understand: Fuel economy complaints/queries? Please copy, paste & answer these questions, esp. if you're new | PriusChat

 

Been tracking my 2010 Gen III since I drove it home in May 2009. Mainly a city commute 18mi round trip weekdays. Also tracked fuel costs. I notice a 5-6% difference in the car MPG and calculated. I also track number of days between fill-ups, 33 days on average. I have around 25K miles 4 years later.

I added a trendline to the calculate MPG and it's -4.5%.

-Jay

 

Over 3 fillups now and my calculated by car has been 48-51MPG. When i do it manually i am getting 51-55MPG.
I was always told it was off to the positive but on ours it seems off to the negative and not sure why.

 

2nd tank on the Used 2010 I recently bought. Displayed 57.4 but the math says I actually got 55.09 mpg. 486 miles . Refilled with 8.823 gallons. After years of 21 mpg (and some far less) I can assure you I'm not complaining one bit. Love this car !

 

Just put my 48th tank of gas in my 11 Prius over 20,900 miles. Over that span, actual MPG (calculated) has been 6.25% below the computer's estimate.

Also tracking differences between using 10% ethanol and 0% ethanol. On average, I use 2.00 gallons of 0% ethanol to drive 100 miles and 2.17 gallons of 10% ethanol to drive the same distance. At current gas prices, 10% ethanol would have to be about $0.30 cheaper per gallon (which it isn't) to be breakeven.

 

Can anyone explain the "logic" of the dash mileage display? I have complete fill up records from the first time through now (last fill up 6/30/13 to correspond with monthly dash display). Over 24,500+ miles on our 2012 the display routinely indicates 2% - 6% more than my actual calculation. However, on three occasions my calculation has been nearly 4% MORE than the display. The only similarities are that, as I always fill up late on the last day of the month, again to have data that corresponds to the monthly dash display, the mileage since the last fill up has varied from 170 miles to 235 miles. Is the dash display somehow "weighted" that I can go from a + to a - in relationship to the dash display? My dealership techies don't seem to have any answer. I'm not complaining, only trying to understand the fluctuations. Incidentally, since my intake manifold was replaced in April (under TSB 0010-12 with part # 17120-37054) we are still getting 4 to 6 miles per gallon MORE than before it was changed. Go figure!

 

If I had to hazard a guess, I would suggest that the occasions your display was greater than calculated might have been because the gas nozzle shutdown prematurely. Was the calculated mpg on the next tank significantly worse than others? In other words, if you averaged the mpg for two tankfuls (one where it was 4% more and the next tankful), did they average out?

 

3 fill ups out of that many total is an anomaly of some point, early nozzle shutoff, bad pump, record error, etc. I wouldn't over think it.

 

The display is always greater than manual calculations, anywhere from 2 to 6%. On the three fill ups where manual calculations were higher than displayed mileage, the next fill up was not out of line. Since I posted on 7/2 I ran across a fellow who does a lot of maintenance, rebuilding, etc. of gen 2 models. His take is that the computer is not "smart" enough to compensate for a tail wind or any type of draft coming from a large vehicle ahead of me. Something about a fuel intake monitoring system that doesn't know how to handle it. Obviously the effects of a head wind reflect quickly on the dash display and the manual calc still stays within the range I have obtained over the past 16 months (the first year the display averaged 4.1% more than manual calculations). I have read priuschat comments from some other owners who must regularly "draft" off other vehicles. I wonder if any of them have had the same type mileage increases? Perhaps one of them will see my question. Thanks for your input.

 

Thanks for your comments, also. My reply is above.

 

Miles: 543.4
Gal's: 10.535
Avg Speed: 39 MPH
Computer MPG's: 53.9
Calculated MPG's: 51.58
Computer over read: 2.32 mpg
Gas pumped on slow speed and first click remove nozzle.
Costco Gasoline, Warrenton, Oregon, Octane 87 Regular 10% Ethanol
Price per gallon: $3.769

 

I don't think I've ever fit 10 gallons into my car, you run it looooooooww.

 

That was actually two fillups and did not reset the trip meter.

 

I see, understood.

 

Where do you find the computer MPG? My first tank calculated out to be only 47 mpg, which was sort of disappointing. It was mostly hwy driving.

 

Under "Trip A or Trip B" If you reset one of them at each fill up it will tell you what it thinks the MPG is for that tank thus far.

Travis

 

Is this found on the navigation screen? (ie under which tab)...I've hunted around but don't see a Trip A or B by the odomoter the way there is in my old Honda civic. Or maybe I am just blind? Thanks!

 

I am talking about the screen closest to the windshield. It has to be on either the "HSI" display or the "Consumption" display to show the "Avg MPG"

Travis

 

Don't be stunned if that is your regular, highway MPG in this area (I'm just over the river in Windsor, VT). Over the 60k miles I have on my 2010, 47-48 ish is my pretty normal highway average. When I avoid the highway and stick to the backroads I can get it up over 50 pretty easily but it ticks back down the longer I'm on the highway (70mph average speed-ish)

Look to the right of the Speedo display. The lower line is controlled by cycling through the TRIP button on the wheel. The display just above it is controlled by cycling through the DISP button.

I reset both Trips A & B each time I fill up (hold down the TRIP button to do this on each) then run the numbers through Fuelly at each fill up. The on-board computer is consistently about 5% optimistic.

Not my car, picture found online \/\/\/\/