MPG variance between actual and computer?

[cavebutter]

Just filled up my "like-new" 2005 for the fourth time. Geek that I am, I'm tracking my mileage. Nothing fancy, just Gallons put into tank, miles driven on previous fill-up, Gallons/Miles, and computer-calculated MPG.

I'm pretty happy with my mileage overall: 48mpg in Los Angeles with lots of AC and driving over 75 on my long daily commute. But I'm noticing significant variance between the computer calculated MPG and Miles Driven/Gallons. This last fill-up was the most significant. Computer-MPG was 49.5, Actual MPG was 47.0. Nothing to sneeze at, but I want it to make sense. Where's the variance coming from?

I am calculating straight Miles/Gallon.
Is the Prius maybe calculating Miles/Gallon per mile driven? That is, is it calculating the fuel efficiency per mile driven?

Anyone have any ideas?
cb

***Edit***
I should have read further down before posting. Just read on an earlier post that 2-3 mpg variance is "normal."

[eagle33199]

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.

[berylrb]

... 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)

[tmgrl3]

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

terri

[Karkus]

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.

[qbee42]

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

Computer-MPG was 49.5, Actual MPG was 47.0.
[/b]

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

[brooksq]

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?

[Ichiro]

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?

[qbee42]

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

[Proco]

Originally Posted by brooksq

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?

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 ...

Originally Posted by qbee42

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

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