Wheel well clearance

[Streamer1492]

How much clearance is needed from the top of the tire to the top of the wheel well? I'm trying to remodel a 58 18' traveler. Currently, there is approx 8 1/2" from the top of the tire to the wheel well and I'd like to borrow about 4 1/2" leaving 4" of clearance. When I measure my 2006 28' there is only 3 1/2", but does a single axle trailer, require more clearance than a double?? thanks Terry