As another example of why RPM is not an indication of how much fuel you’re using, consider the following scenarios involving a 2 year old car:
Tach says 4,000rpm. You’re in 5th gear, cruising at 80mph with a 30mph tailwind.
Tach says 4,000rpm. You’re in 5th gear, cruising at 80mph with a 30mph headwind.
Tach says 4,000rpm. You’re in 5th gear, cruising at 80mph while descending a hill that’s so steep you don’t even have to keep your foot on the accelerator.
In all of the situations, the RPM is the same because 5th gear at 80mph in our hypothetical car will ALWAYS result in 4,000rpm. That’s already been discussed.
In situation 1, the 30mph tailwind means you don’t have to use as much throttle to maintain 80mph, and so you use less gas than you do:
In situation 2, where the 30mph headwind means you have to press harder on the accelerator to stay at speed against that wind.
And in situation 3, since you’re in a newer car, the engine computer actually shuts off the fuel supply as you’re going down hill because the wheels keep the transmission turning, which keeps the engine turning, and so no fuel is needed to keep the engine running, which means your mpg is, until you get to the bottom of the hill, infinite.
The best way to tell if your Acura is more efficient than your Infiniti is to let the experts do the figuring for you. Check the EPA fuel ratings for each car. The MDX has a rating of 15 city/ 21 highway, while the Infiniti gets 16/23. So overall the Infiniti is a more efficient vehicle (but not by a whole lot).