What does the " Z " stand for in 205- 50 ZR 15 ?

It's the speed rating. A Z rating means the tire is spec'd to handle a higher speed. I don't remember that well, and I'm sure on of the experts will chime in, but I think it has to do with the sidewall integrity under speed.

quote Originally posted by Robert 2: What does the " Z " stand for in 205- 50 ZR 15 ?

FYI, courtesy of The Tire Rack.com

"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.
225/50ZR16 in excess of 149 mph, 240 km/h
205/45ZR17 88W 168 mph, 270 km/h
285/35ZR19 99Y 186 mph, 300 km/h "

Have a good evening.

~ Bob

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"Its nice to be important.
Its more important to be nice."

Thanks guys
Robert

The tire rating is actually attributed to a large degree to Nazi Germany, after they built the Autobahn. Obviously at the time tire construction was nothing as it is now, but eventually manufacturers caught on that tire construction had to change to compensate for the excessive speed traveled along the Autobahn. The different ratings correlate largely with the 'safe' sustainable speed that a vehicle can travel. Obviously an old Beetle isn't going to hit triple digit speeds for 20 to 30 minutes at a time, so the corresponding tires were assigned. Whereas later on a Porsche 911 would have easily hit mid triple digits with relative ease.

Tires are tested and tested to ensure that they are safe up to the point of the speed they are rated at. Obviously one could travel faster than the speed rating on the tire, but does one want to take that risk? Additionally, the rating does not guarantee that you would not have a problem at slower speed with a high rated tire (factors such as wheel depreciation with usage which can affect balance could have detrimental effects on the stability of a tire).

You should utilize these ratings though when you can and take note of them. Nowadays, most all tire manufacturers for the aftermarket and even OEM construct tires of high rating, since tire construction is relatively streamlined. Although you as a consumer can still pay particular attention to ratings, as do you really need that tire rated for 146 mph on your Fiero, or something more realistic? That could potentially save money.

It's not just speed. The tires with the supercar ratings tend to also be the tires with the best handling. Not just softer tread, but better belting reinforcement.
Most of the best tires will have a W rating. Y is for up to 186, and (Y) is for over 186.