From the specs table of Michelin XZL tires:
14R20 overall dimensions: 49.5x15.1 w/loaded radius 22.8; spacing 17.1
16R20 overall dimensions: 52.9x17.2 w loaded radius 23.9; spacing 19.5
The minimum spacing is provided in specifications. Means Michelin design tires with ability to be duals.
Guys, if you have 14, 16 or 24R21 XZLs, please measure width of the tread it self, not a overall width. Thanks for advance.