United Instruments Airspeed Indicator measures differential pressure developed between the pitot tube and static pressure system. Indicated airspeed is displayed on the outer scale in MPH and on the inner scale in Knots. 3-1/8" instrument round mount with 1/8-27 ANPT rear pitot and static ports. United Airspeed indicators are available in a variety of ranges up to 300 mph. Specify range when ordering. Meets FAA TSO C2b.