I know that Europe had much higher fuel prices than US but a lot of it was taxes....America found different ways to tax us....My b-i-l was visiting from Europe several years ago and we took him out to my parents place near the beach in Long Island....after paying 5 or 6 'tolls' with the requisite 'wait time' at each toll (this was before EZPass)....my b-i-l stated..."I rather just pay more for petrol like we do".I think the problem is that few American consumers ever pay the world price for fuel. So fuel economy has never been much of a priority for the big 3. If their main customers had been paying $5 to $7 per US gallon for fuel, fuel economy would have been a focus.
In that same vein, why do modern pick-up trucks have to be so big? Compare a 1960s pick up to a modern one. Imagine what sort of fuel economy could be achieved with a modern "1965 Ford F150" sized vehicle.
I do know they now have toll roads over there as well but the difference in cost of living between Europe and here has moved much closer in recent years IMO....'globalization' will do that.