We always assumed that, in order to measure interior volumes, some guy with a protractor, caliper, and measuring tape went and took a bajillion measurements of the interior of a car. We also assumed that this information was then fed to a very fancy computer which, after a minute or two, spat out a number. Apparently, stuffing the cars full of ping pong balls is faster, more accurate, and no doubt more fun.
A Ford Escape, for example, can hold 56,778 ping pong balls. We must say, we're a little disappointed in Ford though. We wanted a picture of an Escape holding all 56,778 balls.