Gauge diameters of 2-1/16", 2-5/8", 3-3/8" and 5" are the common sizes. These are called 2", 2-5/8", 3" and 5"
If it's listed as a 4", it is a 3-3/8" gauge with an over-sized bezel that measures 4" on the outside
2-1/16" gauge, also referred to a 2" or 52mm has a 2-3/8" OD bezel
The 2-5/8"...