A/C Impact on Fuel Economy-A Test

[kwagar]

Due to the unexpected death of a friend I had reason to do a speed run this week from Central Florida to Atlanta for the viewing and funeral.

Background: My smart had 2000 miles on it prior to the trip. My average MPG for all of those miles has been 39.1 with a high of 41.8 and a low of 37.4 per fill-up.

The trip: I left Monday morning and drove to Marietta Georgia. 90% of the drive was Interstate driving. The Funeral Home, Church and my Hotel were all within 1 mile of each other and were just off I-75 so I had no real city driving on the trip. I fueled near my home to start and fueled at home on my return using 93 Octane gas. The full trip required 3 fill-ups and a partial fill on my return home. Total distance traveled was 953.2 miles equally split between the drive up and the return. I filled in Atlanta at the start of my return trip.

The weather was hot sunny and humid with temps in the high 80s both ways.
My speed on the trip in both directions averaged between 65 and 77 MPH.

On the trip North on Monday I ran the A/C the entire trip. The temp was set about 68 degrees and fan was on position #2. My intitial tank delivered 38.4 MPG and the second tank, during the hottest part of the day, delivered just 35.2 MPG for a combined 36.9 MPG on the northbound half of the trip.

I was a bit dissapointed with the MPG on the trip North so I made a decision to leave my A/C off on the return trip on Tuesday to see what difference that might make.

My first tank-full on the return trip delivered 363.5 Miles-the indicator said I had .6 gals remaining but my fill was 8.3 gals when the pump auto shut off. This gave me 43.8 MPG on that tank. I refilled next to my home on arrival after an additional 110.6 miles and it took 2.4 gals for milage of 46.0 MPG.

Overview: On each fill I ran the gas into the tank at about 1/2 flow and shut off the fill when auto shut off occurred. I did not "top-off" the tank at any time. My route was the same coming home as going up and weather conditions were the same. The wind was light from the East both days.

Fuel economy outbound running the A/C averaged 36.9 MPG while the fuel economy on the return without A/C averaged 44.3 MPG. This was a difference of 7.4 MPG or a 17% difference. While I expected to see a difference this was a larger variance than I would have anticipated.

The good news: The trip went very well, the car performed well at speed with no handling issues. Cruise control would have been nice but otherwise it was a more comfortable ride than I had anticipated.

I used a total of 23.6 gals of fuel at an average of $3.94 per gallon for a fuel cost of $92.98. Had I taken our Trailblazer which gets 15 MPG my fuel cost for the trip would have been $228.60 even with regular gas. This was a 59% savings by taking the smart car.

Lesson: For those of you in the North who are tracking fuel economy but are yet to get into the summer heat and use your A/C anticipate a decline in economy when the summer heat arrives.

I love this car and am very happy with the results in spite of the drop off in fuel economy with the A/C on. I just wanted to share my experience and possibly allow you to anticipate the impact you might see this summer.

[smartah]

The one thing that us Northerners do use all year round is the defroster. Had to use mine coming to work this morning because of the high humidity.

[stevenvillatoro]

First of all, condolences to your friend. Next, glad to hear your smart performed expectedly well on your long trip.

Regarding the A/C, I have a big concern about your well-intended methodology. The reason for my concern is the wind factor while traveling north vs. south. I put 30K miles on a Honda Insight traveling between San Diego and Los Angeles (110 miles each direction), and I routinely monitored my mileage extremely precisely. Often I did the 'run' in both directions with no lag time before making the return. I kept all variables as consistent as possible to try to isolate causal factors that might affect mileage. The Insight has a very low coefficient of drag, so its penalty due to wind should be minor compared to many other cars. However, even under light wind conditions, while keeping the A/C constant in both directions, maintaining equivalent speeds, etc., my NORTHBOUND mileage always exceeded my SOUTHBOUND mileage by 4-10 MPG. (Overall freeway fuel economy would range from 60 to 85 MPG.)

I would not be at all surprised if your mileage variation were due primarily to your direction of travel. With the large frontal area of the smart and not-great coefficient of drag, the freeway penalty running against the wind could account for the difference you experienced.

With that said, I'd love to hear from a ScanGauge user who could make the SAME run in the SAME direction under the SAME traffic, speed, weather, and hill conditions... and see what effect A/C usage has on fuel economy.

Thanks for the report, and many more smart miles to you!

[tbranca]

vary nice. the ac doesnt seem to suck any preformance. but I have the cabro. so I just put the top down

[RussBarnes]

My wife always says that everybody knows that it's all uphill from central FL to Atlanta, just look at a map. How can you expect as good an economy going that way?

Sorry about the loss of your friend and hope a little light humor helps.

[jwight]

Not exactly the same subject, but even without the A/C on, I've noticed the fan shuts off momentarily as the car shifts through the gears in [D]. Anybody else getting that? There is something in the manual about situations that cause the A/C to shut off but I didn't see anything about the fan by itself.

[Karl Roth]

Originally Posted by jwight

Not exactly the same subject, but even without the A/C on, I've noticed the fan shuts off momentarily as the car shifts through the gears in [D]. Anybody else getting that? There is something in the manual about situations that cause the A/C to shut off but I didn't see anything about the fan by itself.

lights dim fans shut down how much power does the shifter use?
karl
on the AC issue I think more than just the AC is responsible for the milage difference.
Lets celebrate life

[jefferios]

Sorry to hear the sad news about a friend of yours.

Thanks for taking the time out to inform us with that data.

[helidrvr]

Choose the GPH option on ScanGauge, keep your RPM constant, then turn the A/C or any other load generator on and off to see an instant and precise display of its effect on fuel consumption.

This method does away with all concerns about wind, other driving conditions or driving style because you can take the GPH readings while parked. The only variable which remains would be how hard the A/C is actually working at the time of your sample.

[bumblebee]

I agreed the wind had a lot to do with it. I drove my smart North to Talimena Drive from Dallas a couple months ago and averaged 42mpg. But on the way back going South I only managed 36.5 mpg going against a strong South wind. Had the A/C on the entire time.