Accuracy of trip computer- re: mpg.

[Iceracer]

Just out of curiosity I participated with some friends in an informal economy run.
120.3 miles, gas used 2.3 gal. 52.3 mpg. Computer- 51.2 mpg

[Cobra348]

it is not accurate. I've had it off as much as 1.2 MPG (high)

[EyeQueue]

Yeah, I've been looking into how Ford calculates mpg, and it's a flawed system as far as I can tell. Both while watching the display on my Fiesta and my old Mark VIII, I noticed discrepancies.

First, it doesn't seem to be a "time average" that they are calculating. For example, in both vehicles, the average would drop faster during heavy acceleration than when idling at a stop. Also, the average would never rise as fast as it drops. This leads me to believe that they are calculating mpg per unit of fuel used.

Second, on the Fiesta, they cap the max mpg at 99. Which the fiesta could reach much higher.

Although, I guess that it is always better to underestimate something like mpg than to overestimate it...

[wmussatto]

Originally Posted by EyeQueue

Second, on the Fiesta, they cap the max mpg at 99. Which the fiesta could reach much higher.

I think this is fairly common (2 digit display). The Prius when its going down hill charging its battery (equivalent of engine braking in Hill Mode) displays 99. Since the gas engine is off at this point correct display would be infinity. But that would cause problems.

[FXSTi]

Mine usually reads about 5% higher than I am getting.

Iceracer,

Could you tell us what conditions you did this run under? Things like speed, weather, altitude changes, tire pressure, and load of the car? Also how did you do the fill ups? Did you use any hypermiling techniques, or was this just a straight run with cruise?

Thanks

[B2 SHAY]

Mine has not been reset since the day I drove the car home from the dealer in March. 11568 miles the cars computer reads 39.5 mpg, Fuelly says 39.2.

[Iceracer]

Originally Posted by Cobra348

it is not accurate. I've had it off as much as 1.2 MPG (high)

100% accuracy is not possible. There are too many varables,ie: accuracy of fillup for one.
Another time, on a fillup, the difference was the other way.
But for everday driving and mileage indication, it is pretty close.

[Iceracer]

Originally Posted by FXSTi

Mine usually reads about 5% higher than I am getting.

Iceracer,

Could you tell us what conditions you did this run under? Things like speed, weather, altitude changes, tire pressure, and load of the car? Also how did you do the fill ups? Did you use any hypermiling techniques, or was this just a straight run with cruise?

Thanks

A cold drizzyily day. Temps 50+-. Ran the car just like i do every day.
Altitude ran from around 500 ft to app. 2000 ft and back.
Fillups were until the pump clicked off. The only hypermiling trick was coasting down some of the hills with the transmission in neutral.
Driving speed was around 45 mph on the level.
I find it interesting , that the day before I drove as I always do for app. 90 miles with speeds up to sixty, hill country. Averaged 43 mpg.

[psi chick]

omfg how do you people do this? i can barely get an average of 30mpg, i'm usually 27-28. how the hell do you get such good gas mileage? i've even tried driving like grandma the last week or so and i'm still at 29mpg.

[ibethebear]

I never get less than 35, and on a long trip i get as much as 44 at 70MPH