Hold the debate passengers; and let's get to the gas mileage impact of our choices. Assume we've cleared the Death Valley heatwave inside our car, long-haired passengers have donned bandanas, hats are firmly on, and the electronics will stay off. From the Society of Automotive Engineers (SAE) the paraphrased answer is: 'at highway speed, on a hot day, keep windows up as there's just not that much efficiency gain from turning the AC off. At lower speeds there is some significant benefit'. The typical mileage gain of "windows down/AC off mode" is a few percent for a sedan but can range as high as 10% for trucks and SUV's (see graphic for SUV example). Important note: the left axis on the graphic presented below is gallons-per-mile, not the usual miles-per-gallon. Aerodynamics of body design account for much of the variability between sedan and SUV.
These SAE study charts provide a nice summary, and show the mileage variations at the low and high end of the speed range. Fair warning: extended viewing of the full report may cause non-engineers to slip into a near-catatonic state.
Truth be told, the bulk of our driving is within an hour's drive of home, and a lot of that is well below highway cruising speed. So, should TreeHuggers keep windows down in those conditions? To answer this, we go back to the competing demands issue. Got got a blue-tooth cell phone connected to your car's stero and microphone? Got the hot new voice recognition for your stereo controls? Use your cell a lot while driving? Want a decent conversation with the person next to you? Any one of these conditions provides strong motive to roll the windows up and keep them there. Why would you buy cool new gadgets if you could not show them off after all!
Ever notice the dreaded "beats" that your car produces when a single window is down? The resonating luff sounds produced when a window is down are stressful and extremely irritating. Rolling down all the windows will generally eliminate that effect but there seems to be little quiet in between all or none states unless you've got side vent shades. The more streamlined your car is, the worse the effect of the beats; and, they can start at speed as low as 35 mph.
Opportunities for Mileage Improvement
Consider doing more errands in the evening or at night when its cooler. Added benefit: that time of day it's easier to rationalize stopping at a bar or visiting a friend.
Tell anyone who has a "boomer" that keeping the windows down wastes a huge amount of fuel and is destroying the planet rapidly. That will increase the rate at which they go deaf, providing a partial sonic respite for the rest of us and helping the planet on the way.
When you see someone sitting in their parked car on a hot day, engine running just to keep the AC on, wait until they seem to be absorbed in reading the newspaper or filing their nails. With your own windows all the way down, stare in their direction with big smile until they roll a window down to ask what you need. "Just to share a cool breeze" is all you need to say.
Get a scooter or bicycle.
Paint your car roof white.
Seriously now. Lobby the world's car makers to include solar heat block coatings on all vehicle windows. This stuff exists, it works, and its almost invisible. The effect is to reduce infrared heat penetration of the passenger cabin in the 20% to 40% range. By reducing the solar gain that much, the load on the AC is similarly lowered. By coating all the windows, we could drive cars with glass bubble tops and be cool, seriously digitally cool. Our cars would get by with a lighter air conditioner system, which in turn would lower vehicle weight, increasing mileage even in the winter. Why this has not been done is one of the great marketing and engineering wonders of the world.