Improving fuel economy and reducing noise can occasionally be mutually compatible - the smooth aerodynamics of a 2011 Toyota Prius making it a refined highway cruiser, for example.

However, GM is trialling a technology in the Chevrolet Equinox that makes quietness an outright necessity for economical cruising, using a trick used in high-end headphones.

Noise-cancelling headphones are specially designed emit an "antinoise" sound to cancel out external noise, such as the background hum on a plane. Known as active noise control, sound waves opposite to those of the background hum combine with the external noise to make it nearly inaudible.

So how does this help improve fuel efficiency?

Often, combustion engines are most efficient at low revs when friction is lower and less fuel is being used. A side-effect of this is unwanted noise and "boom" - low frequency sounds caused by resonance at low revs and under load.

GM's Active Noise Cancellation technology, like the noise-cancelling headphones, produces sound waves opposite to the unpleasant engine sounds. Since the sound is no longer a problem for the driver or passengers, GM is free to let the engine rev lower and more efficiently.

The generated sound is linked with the engine's firing frequency, getting around the problem of other external sounds, such as wind and road noise, and sounds of passengers or the stereo system.

The four-cylinder Equinox gets 32 MPG on the highway as a result of this quieter, lower-revving nature - a figure that even beats the 2011 Ford Escape Hybrid's 31 MPG highway rating.