mostly agree with mile high
But the tires that I have found that fit the 18s are higher than I pay for tires on my other car and company vehicles that I maintain. They are an odd size. That being said shop around for a deal. I have had luck on www.tirerack.com at $85 a tire plus $15 per tire S&H as they were over stocked and recently Pepboys got a set of 80,000 mile tires with all the fixins including road hazard and an allignment for around $600.
Actually before I heard about the tire size
needing to be the same I had all different sized tires on it. Same size from factory but from 3/32 to 8/32 and 2 other measures all at the same time. I never heard one peep from the TOD system. I always had the 2 largest on the back and two most worn on the front. I still think it is a good idea to keep it close. if you do get a new one maybe get it shave a mm or 2 but considering the OEM spare is a different size I think this is overhyped.
Others will disagree.
Also I have run premium and regular unleaded. I tried it several tanks full. I saw no difference and certainly not 100 miles more. I doubt that can be accomplished without some other mod or an error in calculation. If it were possible gasoline manufacturers would be touting it and the BO admin would be requiring it. I even ran 110 in it by mistake one time no difference