It's not my big economy project to be honest. Want to keep it essentially stock with a few modifications here and there. If I can get it to average 30mpg(Imp) I'd be happy. The biggest differences to make are in driving style. If I was inclined, a four speed manual would be the next thing, but that push-button auto is too cool. Other options would be an lpg conversion - for when I get a second motor to play with. High compression and a feedback carb should certainley improve £pg, though maybe not mpg.
My economy rod project is the real mpg machine though, and the real reason I am here, still runing calculations and stuff at the moment, here's one of my initial sketches:
Fully enclosed wheel spats and maybe faired in headlamps are other considerations. Power will probably come from a Suzuki G10 1.0l motor hooked up to a Samurai transmission, driving a Honda/BMW shaft drive rear end.
Steel perimeter frame rails and roll cage coupled to a 0.8mm rivetted aluminium body and no doors should keep weight to under 450kgs. Soft top hood faired into the boat tail should also help.
Have a look at a Pembleton three wheeler. 2cv powered lightweight built in Britain that regularly attains 60-70mpg with very little done to the motor. My concept is very similar, but hopfully has better aero and avoids it being yet another Morgan replica.