Yes it's about aerodynamics and rolling resistance and weight and you need X torque to overcome all of that at Y speed, but if you are able to generate that amount of torque from less fuel because your engine manages to extract more energy from the same amount of fuel, you would expect the car to have better fuel economy than its competitor with an engine that has worse thermal efficiency... So unless Mazda is doing something really wrong or the return diminishes greatly past a certain point, I don't understand why they don't have much better fuel economy numbers with an engine that has 56% efficiency (compared to as low as 20% for gas engines just 20 years ago!)