I have had my OEM Goodyear Wrangler RT/S tires siped a few years ago at Discount Tire.
They charged only $10 per tire and it makes a big difference in cornering and stopping on wet pavement.
As most of you know, the Goodyear wranglers that come on the Sport Trac from the factory have terrible wet weather traction. The main reason is the hard tire compounds and the large tread blocks that do not offer much traction. When the tires are siped, it divides the large tread blocks into much smaller tread segments that are free to bend and grip the pavement. These smaller blocks of tread are more flexible and provide much better traction on wet or dry pavement.
There are also reports that siped tires last longer than tires that are not siped, but I cannot say if that is true or not, but the improvement in wet weather traction is very noticeable.
nobleman,
Yes, nearly all tires come siped but some tires have too few sipes, or the sipes are too shallow and wearout in a few thousand miles. The sipes that I had put in my tire are cut in 4-5 rows across the full tread of the tire, as deep as the tread, and each cut is only about 1/8" apart on a slight diagonal angle from perpendicular to the existing tread pattern That makes for thousands of gripping edges to really wrap around the texture in the pavement and give a good grip.
As for having any set of tires siped, that would depend upon how good of traction they have especially on wet pavement. If you have "A" traction rated tires, the probably won't need to be siped, although siping may give even better wet pavement traction. If you have "B" traction rated tires, you will definetly benefit from having them siped.
In either case, you can always try the tires and see how they perform in wet weather. If the traction starts to deteriorate as the tires reach 10K miles or so, you can still have them siped to restore the wet weather traction.
...Rich
