Dear Car Talk:

I recently bought new tires for my 1991 Toyota Corolla station wagon. I had never heard of "siping," but at only $13 per tire and a special "Buy 3, get the 4th free," it was hard to pass up; I bought the siping. The car certainly felt better on the slick streets on my way home from the store, but almost anything would have given better traction than the worn-out tires that I'd had. Do you think there is an actual benefit to siping? And, if so, why don't tire manufacturers do it?

-- Jim

That's exactly the right question, Jim.

Tire manufacturers go to great lengths to research rubber compounds and tread designs. They can make their tires with any tread design they want. Don't you think they'd sipe the tires at the factory if it was beneficial overall?

Well, they do. Winter tires often come with sipes manufactured right into them, and on ice or certain types of snow, those sipes can be helpful. But on summer tires, they're pretty useless -- unless you're driving on a racetrack in the rain.

There's even some evidence that on normal roads, they lengthen your stopping distance on both wet and dry pavement.

And keep this in mind: Having your tires siped also can void your manufacturer's warranty. Michelin, for instance, says "altering a tire outside of its original design immediately voids the warranty."

So our advice would be to get a good-quality set of tires from a good manufacturer, and keep knives as far away from them as possible.