For gas engine, MPG is usually calucalted by using MAF (Mass air flow) PID, because gas engine has fix air to gas ratio (AFR). However diesel engine doesn't have fix ratio so that method won't work.

For Ford 6.0 there is PID for Mass Fuel Desired which can be used for MPG: Mass Fuel Desired (MG/str) * Engine Cylinders / 2(for four stroke engine) * RPM * 60(to calculate fuel used per hour) / 1000(convert mg to gram) / 832(diesel density is 832g per liter)

In L/100kM:
8 cylinder engine(L/100kM): [Mass fuel desired] * [RPM] / [Vehicle speed] * 0.0288461538461538
6 cylinder engine(L/100kM): [Mass fuel desired] * [RPM] / [Vehicle speed] * 0.0216346153846154

Or in MPG:
8 cylinder engine(MPG): 8154.12 * [Vehicle speed] / ([Mass Fuel Desired] * [RPM])
6 cylinder engine(MPG): 10872.16 * [Vehicle speed] / ([Mass Fuel Desired] * [RPM])

To prevent devided by zero in the formula, you can add small number to denominator. Here is the PID file that can be used in Engine Link iPhone app:

ModePID,Description,Min,Max,Units,Formula,Header
010C,RPM,0,16383.75,rpm,((A*256)+B)/4,0
010D,Vehicle speed,0,255,km/h,A,0
221412,Mass Fuel Desired,0,100,mg/stroke,(A*256+B)*0.0625,0
,MPG(8 cylinder),0,100,MPG,8154.12 * [Vehicle speed] / ([Mass Fuel Desired]*[RPM]+0.01),0
,MPG(6 cylinder),0,100,MPG,10872.16 * [Vehicle speed] / ([Mass Fuel Desired]*[RPM]+0.01),0

Engine Link is a highly customizable, inexpensive option for monitoring all of the critical parameters on the Ford 6.0L.

 

That's confusing, I'll stick to using a calculator.

 

X2 just saying

 

So engine link is an "open" app that allows you to write your own gauges? Do they have an apple approved adapter? Wifi, Bluetooth or wired?

 

You are right, EngineLink is an "open" app that allows you to write your own gauges.
The app works with Bluetooth Low Energy adapter (Bluetooth V4.0) and WIFI adapter.
Have this one LELink plugged in my car.

 

The app will do it for you. Just copy the file to EngineLink app and create an average gauge or plot on screen.

 

Just as an FYI:

The PCM provides two key values directly:

miles per hour
gallons per hour

miles/hour divided by gallons/hour yields miles per gallon.
The tricky part is getting a fast enough sample rate to handle the nearly constantly changing values during a non-steady state drive..
This is why on a 600 mile highway drive the SG2 and lie-o-meter are so much more accurate than after 3 weeks of city driving..

I'll just enjoy the scenery and divide miles by gallons with each fill-up.

 

I like your thinking, I'll do one better I'll look at the fuel gauge and fill the tank when she's down to a quarter ( if the bladder can hold that long ). I'll let the wife DIVIDE the roadtrip snacks.