as was stated previously, even with an overdrive installed, be careful you don't lug the COMBINATION at "x"mph where you will be cruising at. that's why i asked feets if he was contemplating a rear gear switch to help the imp at take off, while enjoying the benefit of the od at the targeted cruise speed. the total combination and tune of the entire package, plus the weight and air speed drag need to be considered to get the most out of what you have. using his imperial as an example, it might get better mpg @ 70mph turning 2700rpm than 1900rpm at the same mph using the overdrive in both examples, yet different rear gear sets, just by optimizing the total combination and putting the engine in it's proper operating range for best mpg. just because you are turning a significantly lower rpm at speed doesn't necessarily result in an increased mpg if the engine is somewhat struggling to maintain "x" mph. i hope i have expressed my thoughts properly.