| Re: Z-Rated Tire Question
The size of the tires goes like this:
Section width (in mm) / aspect ratio (%) [speed rating] R Wheel Diameter (in)
Your car currently has 245/50ZR16. That means the width of the tires (at the widest point) is 245 mm (or about 9.6"), the aspect ratio (which is the sidewall height) is 50% of the width, or about 4.8", and they are Z speed rated, Radial construction, and mounted to 16" wheels. A key number is the diameter, which you can mathematically figure out (about 25.6").
Your neighbor's tires are 215/55ZR16. These tires are 8.5" wide, the sidewall is 4.2" tall, and the overall diameter is 25.3".
Assuming that your Cadillac came with 225/60R16 tires from the factory, both of these sizes are much smaller than what originally came with the car (26.6" tall). I wouldn't recommend putting the 215/55 tires on it -- they don't have the required load index for your vehicle and you could be running them overloaded. The same goes for your current 245/50 tires, but probably not as much so.
Look at the load index (should be listed after the tire size). In your case, it'll be a 2-digit number, followed by a letter. The number should be at least 97. If it's less, you could be overloading the tires. | 
Take a hard look at the tire and load information sticker on the rear edge of the driver's door. It will tell you the size, load, recommended pressure, and speed rating of the original tires. The entire vehicle: PCM, speed limiter, wheels, speedometer, transmission, suspension, brakes, and ABS is set up for the original tires. Changing tires and/or wheels involves much more than slapping on a new set.......be very careful. BTW...post your vehicle year and model in your header. It makes diagnosis much easier if we can tell if it's a Cadillac so-and-so or a dump truck.
