GAC Speed Converter NOTE: Mach speeds change with altitude, sea level is 762 mph, at altitude is 660 mph. 40,000 feet is the usual altitude. Please enter numbers only. Choose an altitude:   Sea Level 10,000 feet 20,000 feet 30,000 feet 40,000 feet or higher Enter Mach Speed        or Enter MPH Speed    or Enter Km/h Speed    or Enter Knots Speed    Mach - MPH    Mach - Km/h    Mach - Knots    MPH - Mach    Km/h - Mach    Knots - Mach