Effects of AC on MPG and how to maximize efficiency while using AC

[hybridbear]

Thursday leaving work in Owatonna I got into a very hot car that had been sitting in the sun all day. The outside temperature was 93 degrees. When I got in and started the car the temp display showed 100 as seen below, I had to take a pic of that because it looked neat. I also can see in the picture that I need to get out my California Duster and dust again lol

 

 

Anyhow...I had a mile or so to get on the freeway from our factory in Owatonna so I drove that stretch with all the windows down to help get the hot air out of the car. Once I started accelerating onto the freeway I turned on the AC. Check out the graph below. You can see that the first 2 bars (12 minutes) of driving have lower fuel economy because of the electricity being used by the AC to initially cool the car. Once the car was cooled down the electricity required to maintain that temp is much less and the subsequent bars are much higher. My MPG results for the whole trip at 65-70 MPH on the interstate are below, not bad for running AC at that temp. Those first two bars show what happens when using the AC on short distance trips. When you're getting into a hot car and driving only a few miles you don't drive long enough at the lower power draw to make up for the high initial power draw of the AC. If the first 10 minutes or so come at lower fuel economy while the car is cooling down you'll need to drive many minutes more to bring the trip efficiency back up or else your trip MPGs will be very low.

 

 

 

When it comes to hot weather here are some observations I've made about how to be comfortable while using the AC efficiently.

• When the car is hot and you first turn on the AC set it to a higher temp and make sure that recirculate is on - we usually have the AC set at 72 or 73 to be comfortable while driving, but when first turning it on in a hot car I'll set it to 76 or 77 until the auto climate control drops the fan speed, once the fan speed starts dropping I gradually decrease the temp one degree at a time over the next few minutes until it is down to 72, doing this gets you to the lower power draw state more quickly because then decreasing the temp one degree at a time doesn't increase the load or fan speed
• Try to do without the AC on short city trips - even in the hot humid weather we've been trying to do without the AC on short trips, if you're only driving for 5 minutes the AC isn't going to do a whole lot anyway and will really hurt your fuel economy, it's better to have the windows down
• If you're driving a short city stretch and then on the highway don't turn on the AC until you reach the highway - often times leaving our house we are driving a couple miles and then getting on the freeway, we'll leave the AC off until getting onto the freeway and then turn it on for the highway portion of the journey
• If you're using AC it's better to drive the freeway rather than take the city route - we can view AC power consumption as a rate over time, as long as the AC is running it will draw x kWh from the HVB, when using the AC it's better to take the highway route because then your total driving time is shorter and thus your kW consumed by the AC is less and thus your MPG is higher. For example: to one of our frequent destinations we usually get 55-65 MPG over 6.6 miles depending on the lights taking the city route with no AC, taking the city route with AC drops that to 40-45 MPG, or we can take the highway route using AC as mentioned above only on the miles spent on the highway and get 50-55 MPG (since discovering this over the weekend we won't be driving the city route anymore unless the weather is cool and there's no need for AC)

Hoepfully these tips will help all of us to keep cool in the summer while still getting the most efficiency out of our cars. Please feel free to add additional tips below to maximize efficiency while using AC

Edited July 22, 2013 by hybridbear

[dalesky]

Some very helpful observations! Thanks. I sure have seen my MPG drop since it got hot and humid here. I did have the windows tinted, including a strip on the windshield at the top, and I use the window deflector when I park. I generally use auto start when I am about 50 feet away, and think the hit in gas use is worth it here!

thanks again

[aaronj1159]

• If you're driving a short city stretch and then on the highway don't turn on the AC until you reach the highway - often times leaving our house we are driving a couple miles and then getting on the freeway, we'll leave the AC off until getting onto the freeway and then turn it on for the highway portion of the journey

This has been how I handle my return trip from work, and it only makes sense! This is good advice, as well as everything else.

[acdii]

I'm finding this car is the reverse of any other car I owned when it comes to AC and Highway speeds. I get BETTER with all 4 windows down and sunroof vented than I do with AC on and windows up. A difference of up to 12 MPG depending on where I go. Trip to work with AC on, 38 MPG, trip to work AC off windows down 47-50 MPG.

[MXGOLF]

I Just use the fan with the temp control on the coldest. Click the AC off and it blows pretty cold air out but no AC. Seems to work here in Oregon. My mileage can't get any worse anyway. LOL.