Fuel efficiency highly sensitive to AC settings: My experiment results

If you switch the air conditioning from auto to any other mode, fuel efficiency seems to drop sharply.

BHPian tanmay 2612 recently shared this with other enthusiasts.

I had earlier observed that the fuel efficiency of my Volkswagen Taigun 1.0 TSI MT was sensitive to AC controls. I felt that the FE would drop if I changed from Auto to any other mode. If you switch from Auto to any other mode with even moderate fan speed and temperature, FE seems to drop sharply. Almost by 1.5-2.0 Points per L. Same thing on Highway as well as city.

On a recent trip (+200Km one way) when AC was maintained at Auto with temperature set at 21-22 the fuel efficiency was coming as expected (~around 17-18 kmpl on high way run) but with temperature at 21-22 and blower at level-3 I was getting around 15-16 kmpl. I observed that there is a significant improvement in cooling levels when moving from Auto mode to manual.

However after feedback and comments from fellow BHPians, I decided to to find a good route to check my observations. I finally got a chance last weekend.

I kept the temperature set at 23C and used Auto mode for first half of the trip and then used 23C with manual mode and fan set at L2. I opted to use the morning window when the traffic and signals are not particularly high. Also, temperatures are manageable.

I used the same route to travel onwards and back to avoid any difference arising out of the road, elevation, etc.

Lastly i tried to keep the speed and driving style controlled and avoided any sudden accelerations to avoid its impact on the FE.

Please find the two snapshots, one taken midway of the drive and second on the trip end.

One difference was that I had one additional person while traveling till midway.

Overall observations as per the attached information is that if temperature is kept same and blower is not working too hard, the FE drop is not present. Rather due to some other factor I am not sure which one, I got better efficiency on the return.

For the next step, I will have to try this during peak temperature time to understand the difference more accurately.

Adding to the other topic on fuel. A good quality fuel is surely returning me better performance overall when compared with regular fuel. Currently going with Shell as its located just 2 mins from where I stay.

Here's what BHPian darthvader5000 had to say on the matter:

You're saying its best to leave it at auto? One would think that the lowest rpm for the blower coupled with 21/ 22 degrees or even 23,would be better. What's the auto mode temperature cut off set at, if known?

This is quite an update if true. Fuel efficiency is actually bothersome with price of car and fuel ( though my view is that it is the taxes that are making both look pricey). Am getting around 8 to 10 kmpl in city and guessing around 14 on highway. Definitely want it to be around 12 and 16 respectively.

Here's what BHPian prabhu7944 had to say on the matter:

This is quite an update if true. Fuel efficiency is actually bothersome with price of car and fuel ( though my view is that it is the taxes that are making both look pricey). Am getting around 8 to 10 kmpl in city and guessing around 14 on highway. Definitely want it to be around 12 and 16 respectively.

Check out BHPian comments for more insights and information.