Biggest problem is not the engine in the car but the car itself. So much energy must go into their production, maintenance and running compared to literally any other form of land transport. Take a train, ride a bike, walk, etc. hell even replacing the cars with electric mopeds would make a more significant impact. We need to be reducing the number of cars on the road and encouraging carpooling for commuters who don't have other options (aka mostly folks in rural areas who happen to also be those that will benefit the most financially from driving less)