Hooray! Time for some physics!
There are 4 main things effecting the economy of a car.
1) Friction: The bigger the tyres the more grip. But inversely you will get more friction, which can be attributed to the mass of the car. Air friction, the car has to push the air out of the way. The faster you go, the more are you have to push through quickly. A (very) general rule is to go 30% faster, you need to double your power. This is the main thing that will effect a cars performance and why race cars are streamlined.
2) Auxilaries: Air con, heaters, subwoofers all increase the revs of your engine to support the alternator, and therefore guzzle more petrol.
3) conservation of momentum (newtons 2nd law): Force=mass x accleration. The heavier the car and the more you accelerate, the more force (pull) is needed. Cruisin at a constant speed you only have to overcome friction (newtons 1st law) slowing you down, but to speed up you need more power. hence why braking, accelerating..braking acclerating etc will ruin economy and why city driving is always worse.
4) Engine efficiency: Combustion engines are not efficient by any means. Energy is lost through heat (friction) and noise producing 35% efficiency (electric motors being around 85-90%).The bigger the engine, the more heat loss. the bigger the crankshaft, the more momentum needed to keep it idling. the more cylinders, the more petrol gets burned per rev. and of course you lose further power through the transmission, gear changes, poor tuning etc. Commsirac is right about the engine side of things.
So to conclude, this argument is stupid. There are too many variables and thats what the standardised fuel tests are for. To eliminate these variables. Unless you can account for all these factors, driving styles and terrains, you cannot compare data! I am sure someone nursing a V8 around in the country might get better economy than a hoon in a V6 who does city driving with the air con on and woofers blaring. bottom line, if you want to be more efficient, get a small motor, drive slowly, lose all the weight. i suggest a bicycle.