Besides appearance, the plus concept is a key reason to switch wheels. Plus sizing your wheels and tires is the best way to improve both the performance and appearance of your vehicle. By using a larger diameter wheel with a lower profile tire it's possible to properly maintain the overall diameter of the tire, keeping odometer and speedometer changes negligible. By using a tire with a shorter sidewall, you gain quickness in steering response and better lateral stability. The visual appeal is obvious; most wheels look better than the sidewall of the tire, so the more wheel and less sidewall there is, the better it looks. The idea of plus sizing is illustrated in the photos that accompany this story. Pretend that the four wheels we show you are for the same car, rather than the Focus, Miata, and two 3 Series BMWs they're actually mounted on. Two of the wheels (the Miata and Focus) are 15 inches in diameter, while the BMW 323iT (a wagon) and 328i have 16- and 17-inch wheels. If a car has a 15-inch wheel, then upgrading to a 16-inch wheel would be plus one and a 17-inch wheel would be plus two. You could also say that if a car has a 17-inch wheel (such as many performance cars do) then going to an 18-inch wheel and tire would be a plus one. If the car has 15-inch wheels, the 18s would be a plus three.