How do they measure interior volume?

[Doc]

Here's how Wikipedia says they calculate and index the volume. The index is used to determine the vehicle-class.
http://en.wikipedia.org/wiki/Vehicle_size_class

Interior volume index is calculated differently for different vehicle classes:

* For passenger automobiles, it is calculated for each car line which is not a two seater. For car lines with more than one body style, the interior volume index for the car line is the arithmetic average of the interior volume indexes of each body style in the car line.
* For all body styles except station wagons and hatchbacks with more than one seat (e.g., with a second or third seat) equipped with seatbelts as required by DOT safety regulations, interior volume index is the sum, rounded to the nearest 0.1 cubic feet, of the front seat volume, the rear seat volume, if applicable, and the luggage capacity.
* For all station wagons and hatchbacks with more than one seat (e.g., with a second or third seat) equipped with seat belts as required by DOT safety regulations, interior volume index is the sum, rounded to the nearest 0.1 cubic feet, of the front seat volume, the rear seat volume, and the cargo volume index.

All dimensions and volumes shall be determined from the base vehicles of each body style in each car line, and do not include optional equipment.

Front seat volume is calculated as product of:

* Effective head room-front
* Average of shoulder and hip room-front, if hip room is more than 5 inches (130 mm) less than shoulder room; Shoulder room-front, if hip room is not more than 5 inches (130 mm) less than shoulder room
* Maximum effective leg room-accelerator

Rear seat volume is calculated for vehicles within a rear seat equipped with rear seat belts (as required by DOT), as product of:

* Effective head room-second
* Average of shoulder and hip room-second, if hip room is more than 5 inches (130 mm) less than shoulder room; Shoulder room-second, if hip room is not more than 5 inches (130 mm) less than shoulder room
* Minimum effective leg room-second

For passenger automobiles with no rear seat or with a rear seat but no rear seat belts, the area to the rear of the front seat shall be included in the determination of usable luggage capacity.

Cargo volume index is calculated as follows:

* For station wagon, it is the product of Shoulder room-second, Cargo height, Cargo length at belt-second.
* For hatchback, it is the product of:
o Average cargo length, the arithmetic average of Cargo length at second seatback height-hatchback and Cargo length at floor-second-hatchback
o Shoulder room-second
o Second seatback to load floor height.

[OWC Jamie]

oh come on, they just pull a number out of thier arse

[space-time]

or they could open up a small container with a certain amount of gas (let's say H2) and measure the concentration of that particular gas. The larger the volume, the lower the concentration of that gas. This assumes the initial amount of gas in the air is very low and the car is almost air tight.