Ok, thank you all - I'm going to get my tires replaced.

Cracks in the sidewall of your tire is generally bad from a safety standpoint. The sidewall is the thinnest part of the tire, and cracks make the sidewall integrity suspect. If your be-bopping around town at slow speeds, 35mph or less, a blowout might just be an inconvienience, but at highway speeds, it becomes dangerous, your risking injury to yourself and others to save a few hundred bucks.

That's what I'm afraid of - I read about tires blowing out on the highway causing fatal accidents.

My michelins have bin badly cracked for 3 or 4 years. They don't leak and they run well.

Yea, I never checked before so I don't know how long they've been that way.

depends on the depth of the crack... if they leak then yeah replace them... if you are concerned then again yeah... if they are small cosmetic cracks they should be ok...

I don't think they're leaking. Thanks.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=30 Here is a good link for you..I suggest if under warranty replacing them although not needed in some circumstances..

Thanks for the link - I'll go check it out.

depending on the cracks, your sidewalls may be dry rotted.

how do I know?

I meant, how do I know if it's dry rotted?

it's not normal. have a tire shop look at them. Many tire dealers sell very old tires as new. There is a date of manufacture on them. I bought a new motorcycle tire 6 months ago and it turned out to be 3 years old.

huh, good point. I'll find some place to check it out. I'm wary though of a conflict of interest and someone recommending new ones even if I don't need them.

Even though you have had your tires for 3 years, they may be much older. A tire dealer can look at the numbers on each tire and tell you exactly how old they are and if they need replacing. They should not be cracking at 3 years old.