Alright guys I am a new lexus owner, purchased my car at 38,000 (currently at 50k) and finally need new tires for the car. I am a bit of a noob at the whole tire business and after reading my placard on the car it says that the front is 225/40ZR18 for the front and 255/35zr18 for the rears, now after looking around at prices I can get four of the front sized tires at a much cheaper price than purchasing two and two so that would mean that I would have four tires that were 40 instead of the rears being 35. I guess my question is would this take the performance of my car down or would I be just fine ordering four of the 40's? Or should I just stick with the placard?

 

seems like you have oem staggered wheels set up (front wheels/rims smaller width compared to the rear). I suppose you can put 225 infront and back but the rear will have quite of a stretch on the tires.
hankook v12 225/40/18 $118 each, hankook v12 255/35/18 $148 each (TIRERACK). In my opinion, I would go with the recommended size. Rather not risk your safety just to save a small difference.

 

Stick with factory sizes. mismatch and your traction control might go nuts.

 

You can pick any size you want, as long as all wheels are the same diameter which in this case is 25 inches.
gnawhdivad is correct in which your rims are staggered and will look weird stretching the tire.
Your front rim i think is 8 inches wide and the rear is 8.5inches wide or something like 8.5 and 9 inches (cant remember).

I personally went the square route, and purchased new rims with matching tires all around for the purpose of tire rotation/ longer life.
235/35r19 for summer wheels 25.5 inch diameter
225/45r17 for winter wheels = 25 inch diameter