I have 28-9-14 in front and 28-11-14 in rear with 14" ITP 112 wheels(14x6,14x8)
I can tell you that the 14" tire size runs true to height. THe difference between the 27-28" and the 30" is that the 30" only has 24 lugs compared to the 28 lugs of the 27-28 so they run much smoother. After 10MPH the mega mayhem run smooth as stock. Compared to stock they are an amazing trail tire as they are very stiff with no roll. They find traction in anything, and I found in mud holes I would idle my way through with little throttle input where on stock tires I was wide open. They are not a very wide tire, I was surprised even on 14x8's I don't think they measure a true 11" wide. The front are narrow also. Benefit to this is they really climb, dig well. If there is a bottom, they will find it and pull you out as long as your not high centered.
25" Carlisle vs 28" Mega Mayhem. Same PSI, Off the Bike.
