>> What sound does your motor make? Can you find the base frequency on your 
>> keyboard?
>
> Surely that's just the resonant modes of the exhaust system and the 
> chasis?

Ermm, rev your engine, notice how the pitch of the sound corresponds to the 
rpms?  Some part of the sound will be due to resonances in the system, but 
there will be a obvious frequency component (and harmonics) that are equal 
to the rpm (that obviously change with engine speed).

Actually Formula 1 teams use this exact method to check what RPM their 
competitors' cars are achieving.  From the RPM trace they can estimate the 
power output of the engine (given they know the speed of the car, and that 
all F1 cars weigh the same).

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