The good ol' Auto Doc has received a few letters from readers wondering why the optional alloy wheels on a new car are usually substantially larger (19-inch as opposed to 17-inch, for example) than the base wheels. In theory as well as practice, enlarging wheel diameter (yet keeping the same overall tire height) yields a shorter sidewall. This makes the tire more responsive to steering inputs and road variances (referred to as "feel") because there's less rubber (and compressed air inside the tire) to absorb the energy. Low-profile tires, as they are called, usually come with more aggressive tread designs and thicker sidewalls to reduce tire flex (for better cornering). Why go to the trouble? Optional wheels usually accompany larger engines, bigger brakes and a more responsive suspension to form an entire performance package. One component without the other isn't much good. Share your tips with the Auto Doc at www.shiftweekly.com using the contact form.