if you really care about performance sure its cool to say you get so many miles to the gallon but i dont really care if its 23 miles to the gallon or 16 miles... i get to drive my stang...
No they're not. Unless you filled them up with a gallon and a half of steel shot.
Your method for "computing" the volume only works if you run your car empty on the side of the highway and get your car towed or pushed to the gas station.