I understand that the sensor that fuel flow sensor from Lowrance would function with any Lowrance Nema2000 display, but to get mpg this would have to get info from the gps, it seems this would be more then a flow sensor is capable of doing.
Using the LMF 400 will give you this info, you can display it on the LCX 38 display. But can the LCX 38 give you this info without the LMF 400?

This question also applies to fuel flow coming directly from the engine i.e. ETEC, Suzuki, Yamaha. Which Lowrance unit is needed for performing the miles per gallon calculation with input from the gps?