Windows – To Roll Them Down Or Not

Windows – To Roll Them Down Or Not

People have often asked me, if I keep my windows rolled down or not, and why I roll them down.

There is a general tendency to reach for the AC as soon as you start the car, which I have strongly debated against.

This is my view point after doing some practical research on my car, stretched over a distance of 12 kms ( office to home ) and a drive time of 30 to 45 mins with top speed of 45kmph ( city traffic ) in temperatures of about 30+ ° C

I have done this drive over a period of 20 days and clocked each reading, both ways, in multiple settings, i.e. with the AC on, and with windows rolled down, at the same time period of the journey from Home – to – Work and Back Home.

The Gyan behind the debate and then the tabulated results

Just to Quote, there was this study done by the Society of Automobile Engineers ( SAE) which found that driving with the AC on and windows rolled up, actually only reduced the fuel consumption by 10% while driving with your windows down, decreased the fuel efficiency by almost 20%.

Hard to know how did they calculate and benchmark the fuel efficiency in the first place

That’s why I did my own benchmarking on my Hyundai i20, standard 1400cc Diesel engine.

As a general inference from the data I collected, I found that driving consistently over speeds of 60 to 70Kmph, then it’s advisable to roll up the windows and turn on the AC.

Reason, the engine is already powered up, and is capable of running the AC unit, without having to produce additional power just for the AC compressor. Add to this, at higher speeds, the drag (wind resistance) would be higher if the windows were rolled down, since the wind would enter the vehicle rather than flowing outwards. Besides at higher speeds  the higher drag could destabilize the vehicle compromising safety.

2^nd scenario, if you are cruising in the city, like the testing I did, then it would be advisable to roll down the windows and let in some air. At these low speeds, the engine develops lesser power, which isn’t enough to run the compressor. If the AC is turned on, then there is an additional load on the engine directly resulting in additional fuel consumption. At these lower speeds, the drag coefficient isn’t high enough to alter the stability and fuel efficiency of the vehicle

While you guys would still argue about the best option, the trick is to optimize both situations and alternate between them.

Condition	Week	Day	Kms	Drive Time	Avg Speed	Fuel Consumed	Inference
AC is ON, Windows are Rolled UP	1	1	24	0:45:00	32	14	Turned on the Ac as soon as I started the Car, till I entered the office parking and same while returning
		2	24	0:52:00	27.69
		3	24	0:31:00	46.45
		4	24	1:05:00	22.15
		5	24	1:10:00	20.57
Ac = OFF Windows = Rolled Down	2	6	24	0:51:00	28.24	17.5	Never turned on the Ac throughout the drive
		7	24	0:40:00	36
		8	24	0:50:00	28.8
		9	24	0:48:00	30
		10	24	1:02:00	23.23
Onward: AC = OFF Return: AC = ON	3	11	24	0:45:00	32	16.5	Slightly better than the below figures since the onward drive is slightly uphill, and most a third of the return journey was cruising downhill
		12	24	0:55:00	26.18
		13	24	0:50:00	28.8
		14	24	0:52:00	27.69
		15	24	0:44:00	32.73
Onward: AC = ON Return: AC = OFF	4	16	24	1:03:00	22.86	15	Very Close to the   mileage with AC on, because the Ac was turned on immediately on turning on the Vehicle and the road was uphill as well.