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 divided 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.

 

UMMM, just fill tank completely and then dived miles driven by gallons added

 

If you were to do that, you wouldn't need his $6 app! Took the link out for that little bit.

 

Cute.

 

What you get will be average MPG for a certain period.
What I wanted to share is a way to calculate instantaneous MPG and average MPG, which many folks have been trying to figure out and used to save for Ford diesel.

You can use the app to get instantaneous MPG and average MPG, and a plot.