[lunarhighway]

are you still running the original tire size?.... although that would also offset the odometer so you'de get the same error margin on your manual FE calculations.
or maybe:
some speedometers are reading out faster speeds than you're actially going too, for whatever reason (keep you from speeding... make the car seem faster than it really is?)
but if this data is used to compute your FE this could also account for the difference.

if the odometer is correct than you'd indeed see a difference

just thinking out loud