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
Mach Five (rather than having it serve solely as an excuse for Spritle and Chim-Chim to stow away in the vehicle). The Mach Five is later modified with gadgets and becomes Speed's alternate car for off-track races such as the Casa Cristo 5000, as well as everyday driving like a normal car. The Mach Five originally belonged to Speed's older brother Rex, who relinquished ownership of it to Speed before he left the Racer home. Speed's main car for races on the WRL track (Thunderhead, etc.) is the Mach 6. The Mach 6 was destroyed in a fixed race, but was later