Class sizes are not based on interior dimensions, they are based on exterior dimensions.
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No sir, they are classified on interior space for cars and GVWR for trucks:
http://www.fueleconomy.gov/feg/info.shtml#sizeclasses
The size class for cars is based on interior passenger and cargo volumes as described below. The size class for trucks is defined by the gross vehicle weight rating (GVWR), which is the weight of the vehicle and its carrying capacity. Fuel economy regulations do not apply to heavy-duty vehicles, so they are not tested. See Which Vehicles Are Tested for more information on these vehicles.
Vehicle Size Classes Used in the Fuel Economy Guide
CARS
Class Passenger & Cargo Volume (Cu. Ft.)
Two-Seaters Any (cars designed to seat only two adults)
Sedans
Minicompact < 85
Subcompact 85 to 99
Compact 100 to 109
Mid-Size 110 to 119
Large 120 or more
Station Wagons
Small < 130
Mid-Size 130 to 159
Large 160 or more
TRUCKS
Class Gross Vehicle Weight Rating (GVWR)*
Pickup Trucks Through 2007 As of 2008
Small < 4,500 lbs. < 6,000 lbs.
Standard 4,500 to 8,500 lbs. 6,000 to 8,500 lbs.
Vans Through 2010 As of 2011
Passenger < 8,500 lbs. < 10,000 lbs.
Cargo < 8,500 lbs.
Minivans < 8,500 lbs.
SUVs Through 2010 2011–12
All < 8,500 lbs. < 10,000 lbs.
As of 2013
Small < 6,000 lbs.
Standard 6,000 to 9,999 lbs.
Special Purpose Vehicles Through 2010 As of 2011
< 8,500 lbs. < 8,500 lbs.
or < 10,000,
depending on configuration
*Gross Vehicle Weight Rating (GVWR) is calculated as truck weight plus carrying capacity.