I am just asking to clarify this. Lighter aftermarket rim/tire combinations will take less rotational energy to turn, right? So should one also expect better fuel mileage due to the greater energy efficiency?

 

Even if the rim is lighter than stock, if it is bigger it will require greater angular acceleration to turn, and due to the law of inertia and other principles, energy efficiency would actually decrease. It's not the end of the world though, unless you're at the track.

 

You obviously dont know what angular acceleration is...

 

It all depends on how the weight is distributed on the wheel and tire combos you are comparing. Just because its larger doesn't mean it will take more energy to turn, and just because it's lighter doesn't mean it will take less energy. You have to calculate the moment of inertia. Get out your calculators guys.