Running a higher ratio diff will not necessarily improve fuel economy this is because to run at a certain speed requires the engine to produce x amount of horse power at a certain no of revs if you understand that the higher an engine can rev the more power it will produce so by lowering the revs the engine may not produce enough power to push the car along at the throttle settingused before so now with the higher geared diff you have to increase the throttle setting to maintain the same speed and by so doing you use more fuel The same thing happens when you find your car is faster in fourth gear than it is in fifth because fifth gear is an overdrive gear for economy not power as the civic is a small car with a small engine in increase in the overall gear ratio would only lead to disappointment