That's Mazda's claim about the 2011 Mazda Miata. The engine is code-named "Sky-G", and is a naturally aspirated 4 cylinder engine. The specifics are not known yet about the engine, nor how it is speculated to achieve 70 mpg, however it is believed to be about a 1.3-1.5 liter engine.

The speculation figures of 70 mpg, and more realistically 50 mpg, were conceived by testing a vehicle whose weight and size were comparable to that of the Mazda Kiyora at the 2008 Paris Auto Show.

At the 2009 Tokyo Auto Show, Mazda introduced a 1.3 Liter 4 cylinder engine destined to power the Miata, which would still meet the current power needs for the 2011 Mazda Miata. The engine is designed to create 150 hp, and the expected weight of the 2011 Miata is about 2,200 pounds.

Other automakers, such as Toyota, have tried approaching Mazda about getting details on this engine.

Source: Left Lane News