You can get a "short" 18 inch tire and get a "tall" 15 inch tire, the tire diameter effects the gear ratio more than people think. All these guys running 4.11s or such, I suspect, are running a taller tire, people like me (who are more handling oriented) are running "short "tires and 18 inch wheels to stiffen up the sidewalls for corners. The drag guys run a smaller rim, and a tall tire (large sidewall) this makes the tire flex more for straight line traction. On street cars they may not actually realize this (some do) but unless you drive your car to the extreme, you may not recognize the difference in tire diameter.
Don't get hung up on wheels size, it's the tire ( height)diameter which is most important.
Did you check those calculators I posted?