You should find that info on the drivers side door jam on a placard.
You can always ask Tirerack. They are a club Lexus sponser and you might get a better deal. Something you might consider as well, is the amount of p.s.i you can fill the tire to. my Michelin tires allow a 50 p.s.i inflation. I filled them up to the max preasure and I gained 2 mpg. That is an extra 40 miles per fill up! FREE! I am getting 23mpg now on my SC400! That is without trying. No cruise control and driving @ 75 to 80 mph!