this post was submitted on 26 Oct 2023
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It's because fuel economy is heavily driven by vehicle weight, since start from a stop kills efficiency. Cruise effeciency is more about aerodynamics than weight (ask anyone who's ever towed anything - you can really feel the drag above 45mph).
And oddly enough, today's cars aren't really significantly lighter than 30 or 40 years ago. We've just moved the weight from the frame/body setup to unitized body/frame (lighter but safer... And cheaper to manufacture), more safety systems (airbags/computers) and things like heated seats, etc.
Today's 4+ seat SUV often weighs as much as a 1970's 4+ seat station wagon...but with less space inside.
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!)