Not sure where the flow meter is located for dash MPG calculation, but I'd try to pour a couple bottles of Techron to clean up the fuel path through the flow meter which may restore the accuracy of the flow meter.
There is no "fuel flow meter" in this modern computer controlled system. At least not in the sense of the in-fuel line paddle wheel type sensors used in years gone by.
The fuel flow will be a value calculated by the ecu based injector duty cycles. In simple terms, the ecu knows how much fuel it is directing the injectors to inject which of course is influenced by signals from the myriad sensors in the system. This allows total fuel flow to be derived. Mpg is then calculated based on flow and distance/speed.
Things like dirty injectors therefore would be potential causes of inaccuracies. That, along with new tyres with a slightly different overall diameter and even tyre pressures that havent been checked for a while, dirty engine air filter or even dirty fuel filters, are likely to be contributing factors.
If it was me, I would run a few shots of a good injector/fuel system cleaner thru, check the tyre pressures, and drop a new air filter in. If that didnt improve things, I would think about having the injectors professionally cleaned.
There is of course another variable in this whole deal, and that is the gas station at which you refuel the car. The gas station variables include:
- pump accuracy
- pump flow speed
- the angle that the car is sitting - if the tarmac is flat or at a slight uphill or downhill angle, which will contribute to how full your tank is filled
- whether you fill to the same "fullness" each time.
- how much ethanol they have quietly added to their fuel...
So if you have changed gas stations, or your existing station has recalibrated their pumps recently, or put in new higher flow pumps ....