We are far from the limits of thermodynamics. You'd have to wait till nearer to the heat death of the universe for that to have an effect on the economy.
Only for gasoline engines. And the efficiency limit isn't the limit of possible sources of energy for work. Future engines could be electric, and the power source could be fusion.
With electric vehicles, you reach the problem of fuel weight. There is simply no realistic battery chemistry that comes close to the specific energy of oil-based fuels (J/kg). Gasoline (oxidation) is at 46MJ/kg, while a Li-Ion battery has at best ~0.9MJ/kg or so. Even a Zn-Air battery has a theoretical maximum specific energy of ~5MJ/kg, an order of magnitude less than gasoline oxidation.
You need huge increases in engine efficiency to make up for the extra weight. This is the reason why there are no electric cargo transporters so far - you may not care so much if your personal car is heavier, but a truck or cargo ship definitely cares.
Edit: Perhaps a car-sized fusion power generator is possible in some far future technology, but we are enormously far from that. At a minimum, it would require achieving fusion with materials that don't generate neutrons as a by-product, as those are enormously radioactive, and very hard to shield (read: have to be bulky). Things like deuterium-deuterium fusion are far out of reach at the moment, requiring much higher temperatures than we can currently achieve.
fusion takes is to a straight line to exceeding capacity of the planet to radiate waste heat fast enough to keep it habitable. free energy is self destruction. still better than fossil fuels, though.
electric engines require batteries and batteries are super duper enviromentally expensive to make at scales needed right now, let alone the 10y forward predictions.
