The new rules, which are in effect but apply first to the 2008 model year, mean that the 2006 Liberty's EPA estimate of 23 miles per gallon must improve to 27 by 2011, and Wrangler's 16 mpg must jump to 28.

By contrast, Liberty's and Wrangler's top competitors, Ford Escape and Honda CR-V, have EPA-rated averages of 20 to 24 miles per gallon, respectively, and don't have as far to go to meet their targets of just over 27 mpg.

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How DaimlerChrysler will achieve higher miles per gallon in its vehicles is uncertain. Some improvement could be derived from new vehicle or engine designs and some from lighter-weight materials in the SUV that will not compromise safety.

Still, federal studies find, meeting the higher fuel targets will add perhaps $35 to $40 to the cost of the vehicle.

A key under the new rules is a vehicle's footprint, or the square footage determined by the length of the wheelbase (distance between the front and rear wheel) by average width of an axle.

Liberty's footprint is 44.2 square feet and Wrangler's 41.1 square feet. The figures for the four-door Wranglers and Dodge Nitro, to be made starting this summer at Toledo Jeep, haven't been determined.

Based on that footprint, the Transportation Department uses a formula to set mileage figures. It fluctuates based on the number of vehicles produced.