mpg Inst F
= instantaneous miles per gallon fuel demand*; F is for Fuel or Fool or Fugetaboutit or some other F word.
mpg Avg F
= average miles per gallon since last reset, which resetting is next to impossible to do w/out documentation or a giant hammer; F is same as above.
*Diesel computer programs have a fuel table that determines fuel to the injectors depending on inputs (pressure on gas pedal, position on torque curve, turbo boost and maybe some other variable(s). I think the guage is reporting the speedometer rate/computer-fuel rate.
Mine's busted, the number is too small