DETROIT – Originally dismissed as too small to be sold in the SUV-laden U.S. market, BMW AG’s Mini brand has proven remarkably durable in an era of collapsing sales and fluctuating gasoline prices.

Sales of the brand in the U.S. climbed 29% in 2008, to 54,000 units.

The auto maker introduces at the North American International Auto Show here a new convertible based on the redesigned second-generation platform of the hardtop Mini Cooper, which debuted several years ago.

The new convertible features an evolutionary design and benefits from the new platform’s more efficient engine, which features 22% lower fuel consumption and carbon-dioxide emissions.

New features include an improved electro-hydraulic-powered roof that retracts in 15 seconds and allows the driver to open and close the soft top while the car is in motion up to 20 mph (32 km/h).

Another quirky Mini feature, dubbed the “Openometer,” records the time spent driving with the top down.

A newly developed electromechanically operated rollover bar is situated behind the rear seats and is activated by the car’s electronic safety system.

Engineers improved visibility in the new convertible’s cabin. One of the criticisms of the previous-generation was poor rearward visibility, because the roof did not fold down enough to provide a clear line of sight.

An optional storage space between the luggage and passenger compartments answers another customer gripe: not enough storage space.

The flexible storage system includes an expanded luggage compartment opening; rear-seat backrests that can be folded down and locked into position; and a pivoting, 2-position shelf for packages.

Two engines are available in the U.S. with twin-scroll turbochargers or fully variable valve management. The 1.6L base engine is naturally aspirated and produces 118 hp, while the up-market S version is turbocharged and makes 172 hp.

Pricing starts at $24,550 for the base convertible and $27,450 for the S version, including a $650 destination charge.

The convertible introduction follows the unveiling of the Mini E in November, an all-electric car powered with lithium-ion batteries that has a range of 156 miles (251 km).