LOWEST EMISSIONS is the primary goal of Prius, not HIGHEST MPG.
Heat is needed to make the chemical reaction in the catalytic-converter work. So, the engine is run to create that heat, which means some gas is actually sacrificed for the sake of cleansing exhaust.
As a consequence of that design choice, you will notice reduced MPG whenever the engine is still cold.
In other words, avoid short trips. Take advantage of a warm engine.
Heat is needed to make the chemical reaction in the catalytic-converter work. So, the engine is run to create that heat, which means some gas is actually sacrificed for the sake of cleansing exhaust.
As a consequence of that design choice, you will notice reduced MPG whenever the engine is still cold.
In other words, avoid short trips. Take advantage of a warm engine.