Why then could someone not make a 210hp car that get's 60/40mpg?
C'mon 4mesh,
Take the SAME CAR, the aforementioned Jaguar, and put 210 horses under the hood. It won't make 60/40mpg.
In any mechanical system there are constants. Gravity, for instance, is for all intents and purposes a constant.
A useful cockpit size can be considered to be a constant.
The specific gravity of the components used for mfgr are "constants", in other words even if you halve the size of a steel or fiberglass car you don't actually halve the weight.
Air drag and road friction drag are not constants but their effect isn't "halved" just by reducing size/weight.
Now if you were to actually custom build the Jag from unobtanium such that it's "halved" in every way you STILL haven't achieved your goal. You can't Halve the combustion ratios.
A story:
When I was little our family read tons of stories about monsters and fairies and elves and various phantasmagorical creatures including little people. We all read 'Stuart Little' and 'The Borrowers' about little critters and people functioning as humanlike. One of the things that bothered me as a youngster was the concept of cooking their meals and heating their wee homes using small fires built of wood the size of matchsticks and toothpicks. They'd set for hours around these teeny fires. Well, I'm a Biggle but I can still build a teeny fire!
Only it won't last for hours.
Burn rate. Combustion is a "constant"......... consequently even motorcycles of only 75-100HP with only two wheels and small engines don't gain mileage in a linear fashion as weight/drag/HP drops. A motorcycle with only 1/4 the wt, power and drag of your Jag doesn't get 4 times the mileage.
By the same token some truly HUGE loads like large trucks that're getting only 4-6mpg are actually MORE EFFICIENT than both the Jag AND the motorcycle because they're tuned to operate in a specific range and roads/laws allow that to happen.
This probably doesn't clearly illustrate my point but I still believe it to be essentially true.
al