Originally Posted by NoCones

Check out the page you linked a bit closer.

The R in the tire size is not the speed rating, it's for radius.

Each of the tires he is looking at on his car are W rated (see the 92W after the size), which means they're good to 168mph (higher than Z). For some reason, some tires put the Z in the tire size too...which means they're Z or better...you have to look at the service description (the 92W in this case, to see if/how much better than Z)...from the tirerack page:

"While a Z-speed rating still often appears in the tire size designation of these tires, such as 225/50ZR16 91W, the Z in the size signifies a maximum speed capability in excess of 149 mph, 240 km/h; the W in the service description indicates the tire's 168 mph, 270 km/h maximum speed."

I'd rather have identical tires, but these are so close to one another that I wouldn't sweat it. (assuming they're both close to new...I wouldn't want one brand new and one at the end of it's treadlife.)