Buying tires can be very confusing. There are so many choices, so many specs, and so much advice! Keri is in the middle of a tire dilemma, and came to About.com for help.

Good Morning, I came across your article online, Tire Time, and I was wanting to ask a question of you, please.
I have a 2007 Scion tC, and I need to get new tires. As I have been shopping around, I have been bombarded by differing opinions on what I need for my vehicle. The last person I spoke with (at the Toyota dealer) said I do not want to put anything other than Z-rated tires on my car. I had been looking at a W-rated tire. He said I can “do whatever I want,” but if I get anything other than Z-rated, it will change the entire ride/handling/etc of the vehicle, and that I should really stick with the Z-rated tires.
What do you think about this? I’m very confused at this point, as I’m getting so much differing information from everyone I talk to, whether it be a friend, family member, or auto service person.
Thank you in advance for your reply -- Keri, Missouri

Keri, You're right, opinions about tires are like ... well, you know, everybody has one. The problem with tires is that they are a liability, especially since the Firestone disaster of about 5 years ago. As a result, anybody in an official position won't advise you to do ANYTHING besides what the factory did.
Then there are people like me. I try to tell things from a truthful yet practical standpoint, even if it ruffles feathers (you should see the emails I get from mechanics that think I'm costing them business). Regarding your tires, if you are telling me the correct speed ratings, a "Z" rated tire is rated for speeds "above 149 mph." The "W" rating was added later in response to faster supercars and the fact that the "Z" rating didn't have a maximum safe speed listed. The "W" is rated for speeds up to 168 mph. So, the "W" tire will actually grip the road as well as (and probably better than) the Z-rated tire.
So that's the info. My opinion? Well, technically your car was designed to use the exact tire it came with, so in theory, that tire size will match your car's characteristics perfectly. We're assuming that Scion knew exactly what tire they were going to use when they designed each suspension component. Maybe, maybe not. Anyway, it shouldn't make any difference at all, especially since the W-rated tire has such a high speed rating. Both tires are overbuilt for the Scion, and should be extremely safe. And as long as you keep with the same sizing specs, your ride/handling won't change to any noticeable degree. Hope that helps.