This is the size that is and has been the most standard in the marine industry for tachometers and speedometers such as Teleflex, Medallion,Livorsi Gaffrig I & II, Faria, VDO, Autometer, Stewart Warner & Beede where the size of the instrument housing and size of the dash mounting hole are approximately 3 3/8". Outside diameter of the bezel is 4 1/4".