MPG: Computer vs. Computed

Are you asking for a calibration formula?

Tom

 

Or you asking *more or less* what's the difference between what the display shows vs. true MPG? about 5%, more or less again.

 

The general accepted difference between calculated and displayed is 5%...meaning your displayed will generally be 5% optimistic.

However, I have found that the more city driving that is done (under 55 mph) the less optimistic the displayed is. For 50 hwy/50 city driving I believe the difference is closer to 4%. For 25/75 driving I believe the difference is closer to 3%.

The only true way to tell is to keep a fuel log for your own car and over a year's time track what your difference is based on your normal routes over the course of a year.

 

Such as an offset? Were you thinking perhaps it would always read 2 mpg high or something like that? That wouldn't be possible, because it would mean that you would have some sort of mileage when the car was not moving, which wouldn't make sense.

The error is closer to a percentage, as the others have stated. However, it's not a straight percentage, as the amount will change some depending on external factors.

Tom

 

Has anyone figured out where there is a variation at all? Why can't it just be straight as it?

 

It would cost $1,000 for a dead accurate flow meter for the gas. Think of it as saving $1,000 not as having a lousy mpg calculation.

 

Just a theory, but on short tanks of fillups, under 100 miles I believe the computer is close to calculated MPG's.

Here in Oregon we are not allowed to pump gas in our cars, othewise I would try about 4-5 tanks with maybe 90-100 miles between fillups and see if its is more accurate than say one tank at 400 miles.

My hunch is that the tank mpg will be more accurate on short fillups.

al

 

Bob Wilson's famous Speed vs. MPG chart indicates that the percentage error is about linear with MPGs (so that the absolute error is doubled when MPGs double) (see below).
However, that has not quite been my experience.
When I first got my Prius, I got about 42-43 mpgs, and the mpg indicator read about 2.6 mpg high.
Now I average about 47-49 mpg, but the mpg indicator error has gone down to about 2.4 or even 2.3 mpg.
Any idea why? (One possibility is that the improvement on both counts had something to do with the changeover from winter to summer gas, but I can't quite figure out why that would change the indicator error.)

BTW Alfon--yes, the error seems much more stable on multiple tanks of gas--as few as two. I think that's because no matter who fills the tank, you don't always fill it to exactly the same point, but over more than one tank it evens out. So, if I have a very high calculated mpg on one tank, it's probably because I didn't completely fill the tank my latest fill up (so my high reading is a bit of a fakeout)--but that error will probably be corrected next time (and the calculation will be artificially low).





Bob's chart:

 

My performanceand discrepancies in computed mileage are in my signature. The 2006 is much more accurate.

As far as a relationship Between computed mileage And calculated mileage let me post a few numbers. The first number is what prius told me i had the second is pencil and paper which includes error from the pump. The third number the miles traveled

68.1 Base 62.4 space 622.9
64.2 space 57.7 space 574.6
64.1 space 58.1 space 632.7
Those are the high tanks

51.8. 45 .9. 494.6
52.0 49 .6. 541.5
52.2. 47 .3. 500.2

There you go. Have fun with it