DaimlerChrysler said March 4 that it would make a two-door convertible version of its PT Cruiser in 2004, a development that could pose a challenge to the auto maker's efforts to comply with American fuel-efficiency standards, according to a U.S. official, the Los Angeles Times reports. The National Highway Traffic Safety Administration (NHTSA) classifies the Chrysler brand's retro-looking vehicle as a truck, according to its specifications, which includes an open cargo area created when the rear seats are removed. Federal rules for corporate average fuel economy, or CAFE, require light trucks -- pickups, sport-utility vehicles and minivans -- sold by an automaker in a given model year to get a minimum collective average of 20.7 miles per gallon. And DaimlerChrysler counts on the PT Cruiser, which gets 21 to 26 mpg with an automatic transmission, to offset its gas-guzzling pickups and SUVs in meeting the CAFE rules. A Chrysler spokeswoman said the convertible PT Cruiser would have the same truck-defining features according to NHTSA's definition. However, an agency spokesman was not so sure, according to the Los Angeles Times.