I do believe there's only one way to get +40mpg permanently out of this vehicle. An EV conversion. If it's a hybrid you apparently already have half the parts to do it. Next step would be to fill the vehicle up with more batteries. (It'll be heavier, which is what you want anyway, right?) Add a way to charge them and figure out how to cheat the system so the ICE won't turn on. And Violà! A 45mpg-e vehicle!
__________________
|