Mach number which was also unit-first, and may have influenced the use of the term Mach. In the decade preceding faster-than-sound human flight, aeronautical engineers referred to the speed of sound as "Mach's number", never "Mach 1". Mach number is useful because the fluid behaves in a similar manner at a given Mach number, regardless of other variables. As modeled in the International Standard Atmosphere, dry air at mean sea level, standard temperature of , the speed of sound is . The speed of sound is not a constant; in a gas, it increases as the absolute temperature increases, and since atmospheric
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