The big question would be things such as top speed on road trips, climbing hills and endurance. A former professor of mine spent time posted in New Zealand during the energy crisis of the 1970's. He said the local cars (all imports, about 50% of them American) all averaged 10 mpg or better than their counterparts in America simply because the motoring public demaded it and because the competition from the Japanese was keen.
This was at the same time the American car manufacturing industry claimed said mpg ratings were technically impossible to acheive.
What separates me from a lot of conservatives is that I think government does have a check and balance role to play in fostering competition and encouraging corporate responsibility. The trouble comes not because government has or should have no role, but because the role they play is often to do exactly the opposite.
Democrats are fond about speaking about corporate greed and irresponsibility-- but some of the greediest and most irresponsible activty took place under Clinton when national security was sacraficed on the altar of free trade (or to boost campaign contributions from the ChiComs).
I'd also rate Teddy Roosevelt as one of our ten best presidents because he was the first to use the power of government to curb the monopolistic predatory excesses of industries such as Standard Oil and meat processing.
Unlike morons like Pat Murray and Barbara Boxer who clog the hall of congress today, those in TR's time actually had some understanding of the way business worked and could play a role in minimalist regulation and guidance. Malcolm Baldridge, Commerce Secretary under Reagan, certainly provided some leadership in the right direction with the carrot approach. I beleive proper leadership could do the same in fostering a reduction of energy consumption through improvements in mpg technology.
It is easy to make cars get better mileage.
Just make them lighter and/or smaller.
Of course, making them lighter and/or smaller also increases the chances of you dying if you get into a crash...
Americans are more and more demanding safety. That generally comes at the expense of weight.
A gallon of gas contains only so much energy. It takes so much energy to move mass X up hill Y or accelerate at rate Z. It isn't rocket science. One can track where the energy goes at each step of the way. It isn't like it's being lost in some engineered black hole...
And I assume you know that a British gallon is about 20% larger in volume than a US gallon. Australia likely uses British gallons.