In order to determine the tire bulge, flip a mounted OE-type wheel and tire over so that the brake drum pad mounting flange is facing up. Place a straightedge across the face of the tire and measure down to the mounting pad. Anything that's straight will work-even a carpenter's level such as the one shown in the photo. This figure is the OE backspace with the tire "bulge" included. Repeat the process with a shorter straightedge over the wheel lip only. This is the true wheel backspace dimension (less tire bulge). The OE wheel measurement will only give you a baseline of the available room. In most cars, you'll be able to add more backspace, which in turn allows you to run wider wheels and tires.