Mach Speed Indicator, .3-1 Mach, Aero Commander/IAI Westwind
This is Mach Speed Indicator, or Machmeter, was made by Kollsman for an Aero Commander aircraft as it is made to an Aero Commander part number, 3883063-1. Likely made for an Israel Aircraft Industries (IAI) Westwind (formerly Aero Commander 1121 Jet Commander). Measures ~3.25 inches diameter and ~4 inches deep.
Can't tell how fast you are traveling relative to the speed of sound without a Machmeter!