Answer by
Doug77 (1433)
I feel your friends may have been telling an "in joke".
As you may know Mach 1.0 is the speed of sound (around 650mph, depending on atmospheric conditions). So, Mach 0.15 is one and a half tenths of one Mach [or, about 70 to 80mph].
If that's what they were doing then I wouldn't buy any muffler bearings from them.