The problem is that the speed of sound is not constant in any given medium. It does vary quite considerably with the pressure and the temperature. In the case of aeroplanes, as they go higher, both the air pressure and temperature become less. So Mach numbers do not give an exact speed unless all the surrounding conditions are known. Well it is more than just another name for speed.
Speed is speed.
This is a measure which many people use at some time or other, even if it is only in a casual rather than a calculated way. Fuel consumption can also be stated in another way, by saying how much fuel is needed to make the car go a particular distance. This could be put as gallons per mile. However, unless we were describing the fuel consumption of a jumbo-jet, this figure would be rather small. For example, a car doing 35 mpg uses about 0 . 0286 gallons per mile.