The 16 indicates the tyre and wheel diameter designed to be matched together.
Tyres that have a rim diameter expressed in inches (225/50R16, as well as 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26 and 28) are called "inch rim" sizes, are the most common type of tyre size and are used on most cars, minivans, vans, sport utility vehicles and light duty light trucks.
While not as common, two additional "unique" types of tyre/wheel diameters are still in use today.
Tyres and wheels that have a rim diameter expressed in "half" inches (8.00R16.5LT, as well as, 14.5, 15.5, 17.5 and 19.5) are used on some heavy-duty trailers, heavy-duty light trucks and box vans.
Tyres and wheels that have a rim diameter expressed in millimeters (190/65R390, as well as, 365 and 415) are called millimetric sizes. Michelin initiated millimetric sizes for their TRX tyres that saw limited use on many different car models in the late 1970s and 1980s.
Michelin PAX System run-flat tyres have been introduced as an integrated wheel/tyre system on a very limited basis as Original Equipment (O.E.) An example PAX System size of 235/710R460A 104T expresses tyre and wheel dimensions in millimeters (235 mm Section Width, tyre Overall Diameter of 710 mm and a 460A mm rim diameter, with the "A" in 460A signifying these tyres feature "asymmetric" beads in which the outside bead (450 mm) and inside bead (470 mm) are actually different diameters.
All of these "unique" tyre/wheel diameters were developed specifically because the tyre and wheel design or intended vehicle use required them to be different than conventional tyres and wheels. All of these tyres and wheels feature bead profiles that have a different shape than traditional "inch rim" sizes.
Tyres and wheels with unique rim diameters should never be combined with traditional "inch rim" tyres and wheels.
It is critical that the tyre and wheel diameters are always confirmed to match before the tyre is mounted on the wheel.