Here is how federal regulations describe 'light truck' for CAFE purposes:
(a) A light truck is an automobile other than a passenger automobile which is either designed for off-highway operation, as described in paragraph (b) of this section, or designed to perform at least one of the following functions:
(1) Transport more than 10 persons;
(2) Provide temporary living quarters;
(3) Transport property on an open bed;
(4) Provide greater cargo-carrying than passenger-carrying volume; or
(5) Permit expanded use of the automobile for cargo-carrying purposes or other nonpassenger-carrying purposes through the removal of seats by means installed for that purpose by the automobile's manufacturer or with simple tools, such as screwdrivers and wrenches, so as to create a flat, floor level surface extending from the forwardmost point of installation of those seats to the rear of the automobile's interior.
(b) An automobile capable of off-highway operation is an automobile -
(1) (i) That has four-wheel drive; or
(ii) Is rated at more than 6,000 pounds gross vehicle weight; and
(2) That has at least four of the following characteristics calculated when the automobile is at curb weight, on a level surface, with the front wheels parallel to the automobile's longitudinal centerline, and the tires inflated to the manufacturer's recommended pressure -
(i) Approach angle of not less than 28 degrees.
(ii) Breakover angle of not less than 14 degrees.
(iii) Departure angle of not less than 20 degrees.
(iv) Running clearance of not less than 20 centimeters.
(v) Front and rear axle clearances of not less than 18 centimeters each.
Source: Code of Federal Regulations, Oct. 1, 1998, edition