I am comparing two different tires of the same make/model (Bridgestone Potenza RE760Sport):

225/45R18 91W SL
215/45R18 93W XL

The OE size is the 215 whereas the 225 will fit comfortably. After reading Tirerack's info on their site, I see that the "XL" load rating means that the max load is achieved at a higher pressure of 41psi.

Does this mean the 215 will actually have stiffer sidewalls?

The OE tire is Potenza RE050A 215/45R18 93W XL and I'm pretty happy with it but looking for something longer wearing.

the xl load rating is a reinforced tire which will(almost always) have a stiffer carcass that a non xl rated tire

how ya like the ms3??

Also, the XL's load capacity is rated at a higher pressure than SL (41 vs. 35).

At 35 psi comparable XL and SL rated tires will have the same load capacity.

The above chart correctly shows that an extra load tire is not rated to carry any more load than a standard load tire when both are inflated to the same pressure (up to the standard load tire's "maximum load" pressure of 35 psi). This is because a tire's load capacity is a function of its size (which determines the size of the "air chamber"), its construction (which determines how much pressure can be held) and the actual air pressure used (which determines how many air molecules are forced inside the chamber). All tires with equivalent physical dimensions carry equivalent loads (until they reach their maximum load pressure).

it may also increase the weight of the tire by a pound or two IIRC.

finally a good technical question and you guys answer it before I even see it ....

oh well ... good answers guys

I would add that in most cases an 'XL' tire has better steering response than an 'sl' tire if all other parameters are equal