49 CFR § 523.3 - Automobile.
(a) An automobile is any 4-wheeled vehicle that is propelled by fuel, or by alternative fuel, manufactured primarily for use on public streets, roads, and highways and rated at less than 10,000 pounds gross vehicle weight, except:
(1) A vehicle operated only on a rail line;
(2) A vehicle manufactured in different stages by 2 or more manufacturers, if no intermediate or final-stage manufacturer of that vehicle manufactures more than 10,000 multi-stage vehicles per year; or
(3) A work truck.
(b) The following vehicles rated at more than 6,000 pounds and less than 10,000 pounds gross vehicle weight are determined to be automobiles:
(1) Vehicles which would satisfy the criteria in § 523.4 (relating to passenger automobiles) but for their gross vehicle weight rating.
(2) Vehicles which would satisfy the criteria in § 523.5 (relating to light trucks) but for their gross vehicle weight rating, and which
(i) Have a basic vehicle frontal area of 45 square feet or less,
(ii) Have a curb weight of 6,000 pounds or less,
(iii) Have a gross vehicle weight rating of 8,500 pounds or less, and
(iv) Are manufactured during the 1980 model year or thereafter.
(3) Vehicles that are defined as medium duty passenger vehicles, and which are manufactured during the 2011 model year or thereafter.