Can someone explain to me how the onboard computer calculates MPG. Is it using realtime data of some sort? Is it any where near accurate?

 

I dont know but mine seems to always read 12.4.

 

I am not sure that anyone knows--aside from Ford's engineers. I have read varying theories, but one that sounds the most plausible goes something like this....

The computer checks the throttle position & the engine RPM. It then uses those two values to look-up the fuel injection timing in the fueling table. The computer then averages over time the amount of injector "open" time, and multiplies that by some hidden constant which is the average flow rating of an OEM injector. That provides the average rate of fuel consumption.

At the same time, the computer is also tracking vehicle speed. It can then run an averaged vehicle speed number & the previous fuel consumption number through the usual formula to come up with the mile/kilometer per gallon figure. As incremental MPG values are calculated, they are combined with the running MPG average that is displayed for the driver.


The theory within the theory is that the computer uses averaging at a couple levels to keep the mile per gallon number more consistent/stable. This seems to make sense, as the average of averages would result in more accurate fuel economy numbers with certain driving styles....and a corresponding disparity to other driving styles. Maybe this methodology of calculating fuel economy does not deal well with jackrabbit starts, the folks that are constantly on & off the accelerator (speed up, coast, speed up, etc.), or something as seemingly benign as random idling times. Hard to say.

I mention this because of the wide swing of feedback that can be found between posts regarding the message center (a.k.a. lie-o-meter). Some folks say that their hand calculated fuel economy is within a couple tenths of a MPG, as is my experience. Others report that their numbers are +/-2 (or more) MPG different than that shown by the computer.

 

On my truck the computer is always .8 MPG better than reality. So when I calculate actual MPG for a tank of fuel it is always .8 MPG less than the onboard computer. So it is very consistent.

 

3 tanks I've calculated so far are all .8 high also. I'm about to do fill up #4 and the dash is reading 14.0 on the nose.

 

The processor knows how many injector pulses equal 1 gallon of fuel. It knows approximately how much fuel is left in the tank via the fuel level sensor. Then it does the math based on how many injector pulses vs fuel left in the tank.
That's the simple explanation.
My fuel mileage meter on my F250 and Navigator are both within .2-.3 mpg every time I verify with a calculator. The system works well until you use a tuner that some hack tuning company has configured.
JL

 

I will order F250 4x4 next week. What should I expect gas mileage to be w/V10 truck lightly loaded, no trailer
city and highway?

 

11.5 arouind town, never ran it on the highway without a load.

 

hwy mpg @ 70mph.................................... 14 max.
hwy/city............................................. 11-12mpg
Towing my (9000lbs) 30' fifth wheel @ 65mph....8mpg

If you drive hard the numbers will be lower.

 

Reset you avg MPG when going down a steep grade, with your foot off the throttle. It'll show 99.9 MPG until you get back on the throttle again.

 

I have only calculated mine a couple times but the computer is within 1mpg of what I get... Not too bad I guess... I have been averaging 13.4mpg around town and on the highway empty...