Fuel economy is the ENERGY required to accelerate the vehicle to speed and maintain it.
Power is the rate of change of energy which is just a fancy way of saying acceleration rate, 0-60 times if you need a specific (but transmission and traction also affects this).
But what you feel in acceleration is torque. Not horsepower. Torque is force, rotational force. And force is energy.
So how does this all tie together? Quite easily.
Torque in a gasoline engine is the volumemetric efficiency of the engine. Power is the product of torque and RPM. So if an engine is tuned to have a broad torque curve, the power goes up because the engine can still generate good torque at higher RPMs.
In most cars, engine power needed for highway cruise is less than 15 Hp, including AC. An engine tuned for higher power will not need the extra torque to maintain the speed and will get similar mileage. The engine may be spinning at an RPM where 150 Hp is possible but because it is pulling 16 inches of manifold vacuum, it isn't generating but 15 Hp.