The SAE tested fuel consumption with windows down vs maximum A/C at various speeds using a 2009 Corolla and 2009 Ford Explorer and found windows down was always more efficient in the Ford Explorer and more efficient in the Corolla at speeds below 75 mph.
Quote:
|
At steady speeds between 64.4 and 113 kph (40 and 70 mph), both vehicles consumed more fuel with the A/C on at maximum cooling load (compressor at 100% duty cycle) than when driving with the windows down. The Explorer maintained this trend beyond 113 kph (70 mph), while the Corolla fuel consumption with the windows down matched that of running the A/C at 121 kph (75 mph), and exceeded it at 129 kph (80 mph).
|
https://www.sae.org/publications/tec.../2013-01-0551/
Again that was maximum A/C and these were conventional internal combustion cars with belt-driven A/C compressors so the break-even speed in your Prius should be lower than 75mph especially for moderate A/C usage.
Personally I run windows down until about 45mph. At higher speeds the noise starts to bother me (older cars seem to have much less buffeting & noise with windows down). I'll run windows up with maximum ventilation (A/C off) fairly comfortably until it gets to be about 75* F in the sun or ~80*F in the shade. Old school seat beads help a lot to keep my butt & back ventilated & cool
