Why YSK: People seem to, on average, think that a car takes a lot of fuel to start up. In reality, it takes on the order of a few millilitres of fuel to start an engine. That means if your car isn’t equipped with an automatic start/stop system to stop your engine instead of idling, it saves fuel to turn off your engine and start it back up when you need it.
Caveat: air conditioning and radio might not work with the engine turned off.
Scenarios where this might be useful include waiting for trains to pass at rail crossings, waiting for food at drive-throughs, dropping off or picking people up on the side of the road when they need to load stuff, etc. May not be a good idea to use this while waiting at a red light because starting the engine does take time which would annoy drivers behind you when the light turns green.
Some cars are equipped with systems that will automatically stop the engine when you are idling for a while (e.g. waiting for a red light). If yours is, then manually turning off your engine will probably result in reduced fuel savings compared to just relying on the car to do it for you.
My car has the fuel mileage thing in the infotainment system (2019 Volkswagen)… so I’ve tried looking at it on days when I drive with my windows down vs with the AC on. Oddly enough, on a ~30 minute commute at mostly 50mph, I get better fuel mileage with the windows down and AC off. It’s even more odd because I feel like I drive more “spiritedly” when I have the windows down. It’s not a huge difference, but 2mpg is still better fuel economy.
That said, I still drive with the windows up and AC on most days because summer humidity sucks here.
This is awesome, thanks for the info! Have you noticed a difference if you go faster or slower? Like straight highway driving at like 70 or stop and go city driving?
Driving slower definitely gives better fuel mileage, as long as I’m going fast enough to get it into 6th gear. I don’t measure in stop and go city driving because I do so little of it, and I rarely roll my windows down at highway speeds because I don’t like the buffeting of the wind… so unfortunately I don’t have data on either of those. All my data comes from my drive home on backroads through mostly farmland.