I guess everyone's buying new tires this week:

What is the difference between a 205/50ZR17 and a 205/50R17? Are they interchangeable?

Vehicle is an '08 Mazda3s Grand Touring.

the Z is just the speed rating - Z means it's maximum speed is rated up to 149mph.

for a mazda 3, i can't see how that'd be necessary

and yes, either could be used.

fyi:

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=46

and

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35

Thanks!

You will need to look at the load index after the size if it says "Z"

I.e.

205/50ZR17 89W which means the tire is good for 168 miles an hour.

Regardless you will never see that in a Speed 3, but the higher speed rating generally means the tire offers more aggressive performance.

S = 112 mph
T = 118 mph
H = 130 mph
V = 149 mph
W = 168 mph
Y = 168+ mph
Z = 149+ but load index will specify.

The speed rating is really a temperature rating. Higher speeds = higher temps. But a higher speed rating almost always means shorter tire life because of a softer rubber compound. A softer compound allows the tire to conform to the road surface better.