I found these amazing posts which explain the formulae used to calculate the potential MPGe for any vehicle. They may seem a little complicated on first reading but if you read them a second time it all begins to make sense :-
http://physics.ucsd.edu/do-the-math/2011/07/100-mpg-on-gasoline/
http://physics.ucsd.edu/do-the-math/2011/08/mpg-for-electric-cars/
Any feedback would be useful to other members.