Summer special: How much does your car’s AC affect fuel efficiency and performance?

When I was in Bangalore a couple of years back, driving around with just the little bit of pocket money that my parents sent me, every drop of petrol counted. So, like most other broke people, I always lied to my friends that the AC was not working to avoid looking like a cheapskate. I was told that switching off the AC while driving reduced the load on the engine and saved fuel. It made sense and I never questioned it. But the thing is, I never really knew how much of a difference turning off the AC made to efficiency or performance. While we were going about planning our ultimate Indian automobile AC test, we thought we'd put this theory to the test too. Our test mule was my Maruti Suzuki Ignis petrol AMT long termer.

To find out how much of difference ACs made to fuel efficiency, I did two efficiency runs on the Mumbai-Pune Expressway (the most stable environment we could find). I conducted the first run with the AC on (not at full blast) and the second one, with the AC switched off and all the windows rolled down. I did the runs between 11.30 am and 1.30 pm to ensure that the temperatures during both of them were as close to each other as possible. Since we were trying to determine how much of a difference just the AC makes to efficiency, I had to maintain a steady pace of 80kmph and nothing more, to restrict how much of a part aerodynamic drag played in this equation.

In the first run, with the AC left on, the Ignis returned an efficiency of 24.67kmpl. In the second run, with the AC switched off, the car returned 28.46kmpl. That's a significant increase of almost 15.5 per cent. Of course, this figure is bound to change depending on conditions, driving style and the car itself.

Our next test was to see how much of a difference the AC made to performance. To find out, we did two 0-100kmph runs – once with the AC switched on and then with it switched off. So what was the difference in times? Pretty much nothing! Why? Because most cars these days come with an auto cut-off switch. When you jab the throttle hard, the ECU understands that you're in a bit of a hurry. Hoping to avoid annoying you, it temporarily shuts of the AC compressor to ensure that there is no additional load on the engine, till it finds that you've eased off the throttle. Your blower will still run at the same speed, but the temperature of the air coming out of the vent won't be the same. During our test, we noticed that the air temperature from the AC vents quickly climbed when we accelerated hard, indicating that the AC compressor was shut off.

Almost every car on sale today features an auto cut-off switch. But in the early 2000s, only the high-end cars came with this auto cut-off feature. Which is why, if you're as old as me, you'll remember your father switching off the AC in your Maruti 800 every time you went uphill, and telling you to enjoy the fresh air.

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