over all tire diameter is what you want to maintain. prime example-- stock size on my 93 si vtec is 205-55-15. over all tire diameter was like 24.7 inches. i couldnt get snow tires in that size so i had to get 185-60-15. over all diameter on that was like 24.5". so you need to go to a tire store or go to discounttiredirect.com and punch in your tire size and they will give you a list of plus size tires with the same overall diameter. if you get too big then your car slows down. if you get to small your speedo is fast and you rack up miles faster than actual. good luch