i've got a simpler "efficiency" number in mine. Multiply mpg * mph, and you sort of figure out what the ideal value is, and try to keep it as high as possible while driving. It basically tells your economy in terms of gas and time, which to me paints a more complete picture than just gas economy. (Time "is" money, after all). Finding your max FE is neat and all, but not very practical if it means you're stuck at 32mph in 5th gear.
It's not a physics equation like you're suggesting, but i'm just tossing it out there.
