Originally Posted by bcredliner

As I posted it may or may not increase mileage depending what kind of driving one does--city or highway, the topography and the speed of the vehicle. Fuel choice can make a difference but hardly ever significant. Increases or reductions in performance that I was referring are based on a given engine, not stating the M54 is a performer when compared any other engine. I have a 4.6 that scoots in comparison to a stock 4.6 but it doesn't if compared to a supercar that goes 0-60 in 2.8 seconds.

What often happens regardless of the potential acceleration from any engine is that one uses more pedal to achieve the same acceleration as was used with the lower gear ratio. More pedal means less mileage. Mileage is usually calculated from longer trips on highways where a constant speed over long distance are achieved. One other factor is that the lower RPMS at any given speed can result in less engine efficiency equating to less mileage. Or, especially with lower HP engine more downshifting.