I want to play devil's advocate so here's food for thought.
First I'll state that I haven't seen a technical paper from any manufacturer that describes the function of how their mileage calculation is derived.
Everyone ASSUMES the MPG calculation is an algorithm of the fuel flow and mileage. What if it is a relationship of the fuel tank level and distance traveled? Measuring flow at the fuel line is a futile exercise because all modern fuel injection systems are regulated pressure with a return. Unless both the supply and return are measured and compared it doesn't work. Then there is the cost thing rearing it's head. Fuel trims at the injectors are quite precise, pressure and time are the variable inputs but do we know if the logic has been developed at the commercial level to record and calculate fuel use? I would think in racing programs it is because they don't have tank gauges and distance to empty is a vital number. Once again the cost thing comes into play. There are other things that could be used as an input such as pedal position or throttle body position, the airflow into the engine or the O2 registers at the exhaust.
Then there is the distance, miles, speed input. For years, unless you were driving a police vehicle, the speedometer was loosely set to give an indication that was over actual speed. This also has the effect of giving a higher than actual distance traveled. That in turn affects MPG . Now with modern tools like GPS and Forscan or even the courtesy radar signs on the side of the road that shows you are speeding, these can be corrected to a fairly accurate readout.
Then there are the guys that swear their hand calculated numbers are gospel. This is where it will get good. Do you fill up the exact same amount each time and record the miles traveled? Or do you fill up until the pump shuts off and then squeeze a bit more in to be sure it is full? If you do the latter (or stop when the pump shuts off) how do you know you are filling to the exact same level each time? You see there is a LOT of variance in that method.
As for myself, I adjusted my speedometer / tripmeter to get as close as I could referencing the GPS on my phone and the courtesy radar the township police put up. My mileage went UP by .2 mpg after, about 1.1%, really not enough to declare that an input at fault. My right foot, load carried, even the gas station I fill up at ( I have seen dramatic changes after using some brands, I won't use Royal Farms ever again) or time of year can do that. As for measured fuel consumption, that is an unknown as of now. If I figure out where they are getting the input and how I can monitor that alone then I'll be able to decide if the MPG indication is off. For now I'm content that I can rely on the MPG on my dash and I can laugh at the complaints and reasons why in the meantime!

 

I corrected mine by taking several measurements of hand calculated fuel consumption, same pump, same gas station, using the same fill technique. Only then did I go in to the engineering mode and correct my on board meter.

It is correct to my hand calcs now within .5 MPG, which is close enough for me.

As far as to why the on board is always optimistic, not the other way around, well, that's easy. Most people don't hand calculate, so if Joe Blow A is talking to Joe Blow B about the fuel economy of his new F150, he freely advertises an inflated number for Ford.

 

Just curious. I know I have ran my truck down to about 5 miles till empty and it seemed to calculate my distance better after that. Could be in my mind.

I don’t know how the LOM calculates the fuel used. I do know it CONSISTENTLY states 7.5% less fuel than is actually used with 87 E10 fuel. It states 3% less fuel with 91 Ethanol Free. It is so consistent that I believe it is extremely accurate, just adjusted to make owners feel better about their mileage. (Which can of course be adjusted.)

What I am curious about is if anyone has filled up multiple times the exact amount that the computer has stated it used. If you fill up 29.4 gallons instead of 31.4 gallons, would the computer recalculate or run out of fuel 7% at a time? It would take many tanks to test that theory.

Thanks

 

Its constantly calculating DTE from the constant calculation of MPGs and fuel tank measurement. The MPGs are a rolling average I believe.

 

It's quite simple why the calculation is off. Anybody who has a fuel flow on their boat or airplane knows about the K factor. As discussed in the other thread about how to make your computer read accurate MPG you are simply adjusting the K factor.

 

I found that the longer my truck was a t idle, like warming up before a drive or stop and go traffic, the further the on board meter was off. Seemed to be very accurate on long freeway drives. Of course extended idling isn’t good for fuel mileage either.

As far as how the mileage is estimated by the truck. My guess would be that it is simply a formula based off of miles traveled and fuel injector pulse width. For those that don’t know the pulse width is the duration that an injector is spraying fuel. Fuel pressure remains very steady so fuel flow per millisecond of pulse width is very constant. Heavy on the throttle? More fuel needed results in longer pulse width. Then of course an idling truck would be zero miles per gallon it the gallons used per second of idle time could still easily be calculated.

In summary. The ECM absolutely is capable of knowing precisely how much fuel is being delivered to the engine each and every second based solely off of the total duration the injectors were open.

 

All seem pretty positive in their response but I ask: You know this how? Show us the proof.

 

This is what I found so far. I'll keep digging to see if I can see Ford's strategy to calculate instant economy but it may be proprietary information......

DTE (distance to empty) /Average Fuel Economy (AFE)

The DTE (distance to empty) is calculated in the IPC (instrument panel cluster) using the Running Average Fuel Economy (RAFE), which is the fuel economy over the last 480 km (300 miles), and the fuel level input from the fuel sender(s) to determine how many miles the vehicle can be driven based on the remaining fuel in the tank. The DTE (distance to empty) can vary in the short term by up to 50 miles, but is usually within 10 miles. Even if the fuel economy is relatively constant, the DTE (distance to empty) can be off over a 50 mile range by -24% to +38%. The DTE (distance to empty) display and the fuel gauge both use the fuel level input from the fuel tank to provide their respective functions. If the fuel gauge doesn't function correctly, both the fuel gauge and the DTE (distance to empty) display are affected.

The IPC (instrument panel cluster) defaults to around 18 mpg when the battery is initially connected and changes based on driving habits and conditions. Generally, the driver should be getting somewhere between 13-21 mpg.

NOTE: The actual DTE (distance to empty) can be higher or lower than the DTE (distance to empty) displayed in the message center due to changes in driving conditions. It is important to understand how the DTE (distance to empty) is calculated and the factors that impact the DTE (distance to empty) display when determining how to address any DTE (distance to empty) concerns.

Since the DTE (distance to empty) is calculated and averaged over a longer period of time (480 km [300 miles]), varying driving conditions can have a significant impact on the current or short term DTE (distance to empty) as opposed to the displayed DTE (distance to empty). This difference often leads to customer complaints of incorrect or invalid DTE (distance to empty). The following list provides some (not all) of the driving conditions that may lead to an incorrect or fluctuating DTE (distance to empty) concern:

• Changing between towing/not towing
• Changing driving between city and highway
• Allowing the vehicle to idle for long periods of time
• Using the remote start feature frequently to allow the vehicle to warm up, particularly when parked on a grade
• Parking or driving on grades
• Inconsistent use of gasoline or E85 fuels
• Over-fueling or not filling the tank completely (partial refueling)

To better illustrate the affects of how driving conditions can affect DTE (distance to empty) , refer to the following 2 examples. The first example below illustrates how the following observations are normal and expected since the low fuel reminder is triggered from a fuel volume and not from a fixed distance to empty.

• If while driving, the low fuel reminder (low fuel indicator and low fuel warning message) displays when the DTE (distance to empty) equals 94.4 km (59 miles) and the driver adds 11.36 L (3 gallons) of fuel, the new DTE (distance to empty) may become 124.8 km (78 miles). After continued driving, the low fuel reminder may now display when the DTE (distance to empty) equals 83.2 km (52 miles).

The second example (below) illustrates what occurs when idling on an incline. In this example, the customer should be made aware of how the condition will correct after a few minutes of idling on a level surface.

• If the customer stops and parks the vehicle on an incline in a driveway, then in the morning remote starts the vehicle, allowing the engine to idle, the DTE (distance to empty) may now equal 184 km (115 miles). As the customer drives, the low fuel reminder displays when the DTE (distance to empty) equals 148.8 km (93 miles). Finally, after 5 more minutes of driving, the DTE (distance to empty) is back to 80 km (50 miles).

 

Does anyone know how to compensate for this with forscan. Also, in a couple of weeks I'll be swapping my 23 gallon for the 36 gallon. I've never used forscan before, and was hoping someone could give me some tips

 

The proof is pretty easy, although indirect. The vehicle runs. If those things were not true the vehicle would simply not perform well at all. Also whether you believe the fuel used is accurate it is consistent. Therefore the measurements and metering have to be consistent. The metering and measurement of fuel used and the timing of the metering is the essence of the performance of the fuel system. Then because of this MPG can be calculated. If you dismiss this then I doubt you would accept any sort of proof.