Quote Originally Posted by Spenceratc7 View Post
Just a personal story that will back up everyone else here. I went to a local shop down here and threw on 20's for my S4, and with my ignorance and lack of knowledge in this area
I had tires put on with them that were quite a bit under the Load specifications (and the shop did take a bit advantage of me by only making sure there was no rubbing) Needless to say I am lucky that nothing serious happened, but after 4,000 miles of daily driving I had the metal sheet coming through the tire. I am very lucky I did not have a serious problem. If safety doesn't concern you then financially you will throw your money away elsewhere. Tires will have to be replaced more frequently. It will have a major impact on all of your suspension components, not to mention the quality and performance of your ride will be greatly diminished.
Ok. What was the load index of the tires you got, and what load index is required for our cars? Do you know the load index was too low? Are you sure that's the reason for the tire wearing out so quickly?

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Quote Originally Posted by ebarberena View Post
Taking only load rating into account, you do not need XL tires on a S4 B8. You do need at least a 89 load index tire.

An S4 B8 can weight up to 2,300kg loaded (1800 kg for the car, 400kg for 5 x 80kg passengers, 100kg for 4 x 25kg language) or 575kg/tire (with the car loaded this way I would estimate a 50/50 weight distribution).

So we need tires with at least 89 load index (580kg). Most tires that fit our cars have a load index of 92 (630kg) or higher.

As reference, my OEM tires were 245/40-18 93Y

That said, a lot of people believe (me included, but I don't have any facts to prove this belief) that sidewall stiffness is related to (but not only determined by) load rating. The higher the load rating, the higher the sidewall stiffness for a given tire design/size/aspect ratio.

Here is a little info about XL nomenclature on tires http://blog.tirerack.com/blog/bens-b...s-xl-stand-for . You can also look up your GAWR (my car is none US spec so I don’t have GAWR on my door placard) to double check.

By the way, 1kg=2.2lb.
Finally someone replied with some good info :)