In a diesel car the instantaneous mpg readout fluctuates wildly even with minute 'throttle' pedal movement - it is a useless measure for any practical purpose !!
I only ever use the avg readout.
Wind resistance/drag coefficient will vary between car models - mine is a fairly draggy shape which is why I lose 2 mpg on a long fast motorway run...in previous and more sleek cars - long distance motorway driving has actually given a better mpg.