MPG variance between actual and computer?

Yeah, a couple of MPG is normal. The computers control to a fairly high degree the amount of gas that is put into the engine at any given point in time, making the MFD's number highly accurate. Most of the differences come from the bladder, as it can stretch or shrink between fillups, leading you to having different numbers. There's also pump variability... all pumps are supposed to be the same (or at least within some tight tolerances), but i think we all have seen pumps that were clearly not within those tolerances.

 

... and, lately i've been filling on the slowest setting and noticed a difference the other way, 2-3 more than MFD. Of course, the first time using the slow setting doesn't really count, but subsequent fills for 2 months now seem to suggest a pattern. (same station)

 

Third tank of gas... approx. 2 mpg higher on MFD.

terri

 

Due to the bladder and differences between gas pumps, you'll see large errors of up to 5 mpg when calculating mpg from gallons and miles (compared to the computer) but if you track it over a long time, then that method is very accurate of course.
I've got 46000 miles, and a big spreadsheet with all the data. I think the computer is much more precise and usually a better indication of MPG for individual tanks. However, over 46000 miles, the computer reads 1.5 mpg too high (compared to the 46000 miles/ total # of gallons put in the Prius). So for me, the most accurate per tank mpg is to take the computer readout and substract the 1.5 offset.

 

<div class='quotetop'>QUOTE(cavebutter @ Oct 3 2006, 01:51 AM) [snapback]327196[/snapback]</div>

The actual MPG was 49.5, your calculated mileage was 47.0. The computer does a way better job than you can do over a few tanks. The bladder adds a big noise term that can only be eliminated by summing over a very large number of fills.

Tom

 

i've just read the above posts....i see that 2-3 variance sounds normal but what about 6/7. my first fill up posted 48.5 mpg on the computer but the actual was only 42? still normal?

 

so how does the computer compute mpg? from the duty cycle of the fuel injectors & RPM integrated over time? flow sensor in the fuel lines going into the injectors?

 

Read them again. You can't do meaningful manual fuel calculations on single tanks of gas. The bladder system in the Prius introduces too much variation. If you want to do manual mileage calculations you need to do them over a large number of tanks. I wouldn't consider anything less than ten tanks.

To put it another way, each time you fuel it's a crapshoot as to how much the tank will hold. Say you have burned nine gallons of gas since the last fill, but when you refill the tank only takes eight. When you do your mileage calculation the gallons part is off by a whole gallon. Next time you may burn eight gallons but the tank will take nine. You just don't know. All you know is that it all evens out over many tanks.

Tom

 

I've seen what you describe many times. I'll have one fill-up where the MFD says 53 and my calculation comes to 47. The following fill-up the MFD says 54 and my calculation comes to 60. That's why what Tom says applies here ...

Over 50000 miles, my calculations are running between 1 & 2 MPG below my MFD.

 

More confirmation necessary?

After just 10,000 miles, my MFD readings show a (lifetime) difference of 1.5 MPG higher when compared to the "actual" readings miles traveled vs. gallons of gasoline purchased. The parenthesis around actual are intended to emphasize the variability in pumps. I have a difficult time believing these pumps are accurately measuring and delivering to the 1/1,000th of a gallon of gasoline.

So, I am at 50.0 MFD vs. 48.5 "actual".

 

1.5 MPG higher is consistent with what others have reported. The MFD is usually 1 to 2 MPG higher than long term manual calculations. That puts the MFD within about 3%, which is quite accurate considering the small amounts of fuel in each injection.

As to pump accuracy, how accurate do you think your manual pump measurements are? Gas pumps at stations have an allowable amount of error. Guess which way they almost always vary. In reality your 1.5 MPG differential is probably less due to the error in the gas station pumps.

Tom

 

I'm new, only three tanks of gas so far, but see values about 4 MPG lower in actual than computer based. (Computer tells me 48, I am getting 42-45). I will see over a longer period....

 

desired... no

expected... hope not

unusual??.... afraid not

forget the first tank. the dealer filled it. who knows how?... track all your tanks... the two numbers will start to merge. my "calculated" numbers are low compared to the Prius computer numbers but i knew they would be. having had a Prius before, i knew there would be a difference since i lost my first pip after like 35 miles after picking up my 06. a true full tank would have gone 80-110 miles or so. and yes, my first tank difference was pretty large...

 

Quite! That's what I've been thinking. The gap between MFD and "pump" has narrowed over the past nine months, but it seems to be pretty consistent. I've guessed that gas station pump variation and "reliability" (in terms of the actual amount pumped) likely account for much of the 1.5 MPG difference.

 

I've always filled at the same gas station for 3666 miles (approx 15 fill up). MFD average was 37.25mpg and my manual calculations 37.45mpg.

But if I compare MFD to my log at each fill up, I sometime see some quite different mpg value.

So it seems as if for me it's pretty close but I'll keep watching after more miles.

 

I've constructed a spreadsheet to keep track over the 10,500 + miles since I've owned my Prius.
The mileage obtained by dividing the total distance traveled (odometer reading) by the total gallons used (as indicated by the gallons shown on the gas station pump) is 48.27 mpg.
The mileage obtained by dividing the total distance traveled (odometer reading) by the fuel consumption indicated by the MFD (as calculated by dividing the miles driven since the last fueling by the mpg shown on the MFD at each fueling) is 49.49 mpg.
So, in my case, the average mpg displayed on the MFD is optimistic by just over one mpg, or optimistic by about 2.5%.

 

One or a small number of fillups will not give an accurate MPG figure: the tank bladder has variable capacity so the amount you burned since the last fillup is usually not the same as the amount you put in at the current fillup. Long-term record keeping by many people reported in this forum shows that the MFD MPG value is always within a few percent of the manual MPG value.

Interestingly when they disagree the MFD MPG values are usually larger than the manual MPG values. Possible explanations include:
- the MFD exaggerates a little to make us feel better
- the typical gas station pump cheats a little to make more profit for the gas station
- some fraction of the fuel evaporates before it can be used

 

in 2005, a news station commissioned an independent study of gas pump accuracy (this was in response to impending TWO DOLLAR GAS!!) they tested over 200 pumps at like 60 some stations and around 30% were inaccurate.

of the inaccurate pumps. 80% favored the gas station. so if the pump is inaccurate, it will most likely be cheating you...

either way... this discussion is a non-issue imm. i will always take the MFD reading as more accurate because it simply is. i track both for reference purposes only and back a few years ago, i noticed a large difference in the pump calculations (which in itself is not unusual) but i was 4-5 mpg short 3 times in a row at the same station and it hadnt been calibrated in 3 years (state mandates every other year).

i sent a letter to the State Dept of Measures... it took 6 months for them to respond and the letter stated that the pumps were off, had been fixed but the discrepancy was not significant enough to warrant any further action...

all very fishy to me since yes, they were the cheapest station in town...after that fiasco, i started going to Costco and because of the 3% cash back on the Costco credit card, it is now by far the cheapest place in town for me now.

 

I also have been tracking my MPG with a spreadsheet. Yes the more you fill the more accurate the two readings will come together. What I see is the way you may fill the tank. Unless you see the gas at the top of the fill tube, you may not be filling it the same each time. The MFD is getting it's readings from a flow meter and does not read the amount of gas in the tank. That is what I think anyway. I work with flow meters. That is why the MFD is more accurate for single fills.

I do have a question that maybe one of you guys can answer. In tracking my mileage I have found that the Odometer is about 10% different than the Trip Meter or the MFD. The Trip Meter and MFD are the same when I reset them at the time I fill the car with gas. I have the spreadsheet totaling the trip meter readings for the total miles driven. I noticed one day that the Odometer wasn't the same as the total..

Any ideas out there???