It all depends on the tire. 305/25/18 would have less sidewall than 305/30/18. That middle number will be what determines how much sidewall there is. Take the 305/30/18 for example. The middle number is your sidewall height, the 30 meaning 30 percent of the tire width. In this case, it means the tire's sidewall is approximately 90mm because .30 x 305 = ~90.
Sent from the Death Star.
2012 Kona Blue 3.7L V6:
Borla LT headers, O/R h-pipe, Borla S-types, 3.73 gears,
Steeda adjustable UCA w/ mount, BMR LCAs, Airaid CAI,
LCA Relocation brackets, AE driveshaft, GT 80mm TB,
Black 18x9/10 2010 GT500 Wheels w/ Nitto NT555s,
Eibach Sportline Springs/shocks & Koni STR.T Struts