The use of light-weight alloys in wheels reduces rotational mass. This means that less energy will be required to accelerate the wheel. Given that each pound of rotational mass lost provides an equivalent performance gain as a 10 pound reduction in vehicle weight, the benefits of light alloy wheels on vehicle performance cannot be overlooked.

Vehicle        Combination        Average Speed        Average mpg
BMW 3-Series equipped with
Original Equipment (16")        Stock        38.7        22.8
BMW 3-Series equipped with
Plus One (17")        Lightweight        39.0        21.5
BMW 3-Series equipped with
Plus One (17")        Heavyweight        38.8        21.1

Shaving a pound from your tires is equivalent to shaving at most 2 pounds of non-rotating weight. That's PER TIRE, so a pound off each tire could worth close to 8 pounds of weight reduction. For wheels, the multiplier is closer to 1.6, so saving 5 pounds per wheel (20 total) would feel like a static weight reduction of 32 pounds. For brake discs, it can be as low as 1.2.