While the MINI’s Tritec engine may not be known for it’s effeciency, it was still enough to best the rest of the US convertible market in terms of fuel economy. Here’s an excerpt from the MSNBC article:

Mini’s Cooper convertible may be leading the automotive industry in cheap thrills. It’s sporty without being at all obnoxious, its droptop allows you to enjoy nice days–and that gas mileage makes the experience guilt-free.

[ Pump Busters 2005 ] MSNBC.com