I think you have to treat the instantaneous mpg figure with caution, whether from a dash readout or an OBD device. Drivers will concentrate on getting a 'good' number from the readout which may not be good for economy.
For example, very slow acceleration that gives the highest mpg figure is, despite everyone's gut feeling, not the most economical way to accelerate. Best practice is to use wide-open throttle at medium revs (around peak torque) as then the engine is then working at maximum efficiency. It's like removing a band-aid - a short sharp pull hurts less than a long slow reticent one. If you want confirmation, look at economy competitions where 'coast and burn' techniques (maximum acceleration followed by coasting with engine off) give the best results - not a good idea for road driving!
Similarly the best mpg reading is obtained when doing the single most uneconomical thing in driving - using the brakes