Got a question. I also was searching 245/40R20 wheels but my tire shop said 245 tires are usually rated 95 and I need XL tires for my Model S. That guy recommend that I choose 255/40R20 instead to get XL rating. What do you think?
Your tire shop is speaking of the load rating (LL, SL or XL), as well as a related parameter, the service description of the tire. The critical parameter is the service description, which consists of a 2- or 3-digit number (called the load index) and a letter (called the speed rating), usually shown right after the size of the tire on the sidewall. Whatever tire you buy, it needs to have a service description that is equal to or exceeds the original manufacturer's tire fitting.
The original tires on the car are a Michelin Primacy MXM4, size 245/45R19, service description 98W, load rating SL. The service description of 98W indicates that the tire is rated at a load index of 98, which corresponds to a weight-carrying capacity of 1653 lbs, and a speed rating of W, which corresponds to a maximum speed of 168 MPH. The load rating of SL indicates standard load, meaning that the tire can carry its rated 1653 lbs of weight as long as the tire pressure is 35 psi or above.
Let's look at a tire in a 20" size, the Pirelli Cinturato P7 All Season Plus, size 245/40R20, service description 99V, load rating XL. The increased load index component of the service description (99) corresponds to a higher weight-carrying capacity of 1709 lbs, but the speed rating of V corresponds to a lower maximum speed of 149 MPH. The XL load rating means that the tire can carry its rated 1709 lbs of weight as long as the tire pressure is 41 psi or above.
This tire would be fine for use on the Model S, as the load index is greater than the manufacturer's original tire fitment (98 = 1653 lbs), but your minimum pressure for this tire is higher -- you should not inflate is less than 41 psi to ensure that the tire can carry its rated load.
There is no need to get any specific load rating for the Model S as long as the load index in the service description is at least 98. You can use a 98 load index tire that is SL or XL load rated, just pay attention to the minimum pressure required.
I have fit my car with two other sets of tires:
Winter: Nokian WRG3 245/45R19, 102V XL. 102 = load carrying capacity of 1874 lbs, V = speed rating of 149 MPH, XL = requires 41 psi minimum pressure.
Summer: Michelin Pilot Super Sport 255/40R20, 101Y XL. 101 = load carrying capacity of 1819 lbs, Y = speed rating of 186 MPH, XL = requires 41 psi minimum pressure.