Mach number temperature generally decreases with increasing altitude between sea level and , the speed of sound also decreases. For example, the standard atmosphere model lapses temperature to at altitude, with a corresponding speed of sound (Mach1) of , 86.7% of the sea level value. While the terms "subsonic" and "supersonic", in the purest sense, refer to speeds below and above the local speed of sound respectively, aerodynamicists often use the same terms to talk about particular ranges of Mach values. This occurs because of the presence of a "transonic regime" around M = 1 where approximations of the Navier-Stokes equations used
Ford Mustang Mach 1 sought to revive the Mach 1 name. While similar to the Bullitt in the use of the Cobra's Brembo front brake rotors, unique Tokico gas shocks and struts, and lower and stiffer springs, the Mach 1 received a huge performance gain over the base GT and even the Bullitt in the form of a unique variant of the DOHC 32-valve 4.6 Liter Modular V8. Commonly known by Mach 1 owners as an "R" code DOHC, (for the unique VIN engine R code) this all-aluminium engine features the same high flow heads as the 2003–2004 SVT Cobra, 2003–2004 Mercury Marauder, 2003–2005