I've been doing a similar test the last week and a half. Drove hypermiler style and got my 400 mile tank. After that I drove not so hypermiler but still tried to cut back on A/C use and got 42mpg. This last tank I've been going mostly A/C - limited non-A/C use. My Scangauge jumps around on calibration but it's reading 40mpg right now and it did take some doing to get it up there while using A/C.
Most literature on automotive AC puts draw on the system at around 5 HP. The compessor (the power consumer) is clutched and locks on demand and freewheels when demand is met. Freds 6-8 mpg penalty seems excessive for a 5 HP demand. Even if the compressor was unable to keep up with demand and had to run constantly (low charge situation).
karl
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.
Just a question relative to the AC (and an east gentle breeze is NOT going to affect drag coefficient while traveling north or south).
When you had the AC on while traveling north, was the roof shade open or closed? AC load with all the area the smart has open to heat radiation from the sun (even with the smoke PC plastic roof), that parasitic could have made quite a difference insofar as how long the AC was ON for the whole trip, you know?
Most literature on automotive AC puts draw on the system at around 5 HP. The compessor (the power consumer) is clutched and locks on demand and freewheels when demand is met. 12 = 17 mpg penalty seems excessive for a 5 HP demand. Even if the compressor was unable to keep up with demand and had to run constantly (low charge situation).
karl
Thanks for pointing out my blunder have modified my post still "feel" the penalty is higher than it should be. 90 degree weather should not send the AC screaming for help if all is in order. Do you "Think" otherwise?
karl
Total driving resistance = rolling resistance + aerodynamic drag + climbing resistance
Rolling resistance factors are: pavement smoothness, load, speed, and tire pressure.
Aerodynamic drag factors are: head wind velocity, vehicle speed, drag coefficient, and cross sectional area.
Climbing Resistance factors are: gradient of road (+ for incline, - for downhill)
The formula for aerodynamic drag is proportional to (vehicle velocity + head wind velocity)^2 (squared). In other words, it increases EXPONENTIALLY with vehicle and wind velocity added together. The velocity factor at 75 mph is therefore about 300% to 400% higher than at 50 mph. That value is multiplied by the drag coefficient, which is terrible for convertibles and increases with windows down over 40mph. Drag coefficients are in the range of .15 to .7 for passenger cars so I am not saying your mileage decreases 300-400%, obviously much less depending on the vehicle.
I don't think you can shrug off the climbing resistance of a long trip where elevation increases (like going from the coast to the mountains). Think of it as the energy it takes to lift the vehicle (1,600 Lbs + load+ passengers) the same height. Picture an elevator powered by your smart's engine lifting itself to that elevation. Also, going up and down a series of rolling hills with a gradual elevation increase is going to cost you dearly because you are, in effect, raising the gross weight of the vehicle over and over again but there is no way to put that energy back in the tank when you go downhill on the other side of the crest. You get it back in velocity but not gallons of premium gas.
So, some of these factors we control and some we don't. Velocity is the number one controlled factor. Tire pressure, weight and windows up are the minor controlled factors. You have to ask yourself how important it is to get where you're going fast. If you spend an hour in the car at 75 mph you will get there about 9 minutes faster than if you drove 65 mph. But that 9 minutes cost you a lot of gas, tire wear, and stress on the engine. Is it worth it? Up to you!
Last edited by WickedMessenger; 06-09-2008 at 02:11 PM.
Just to pick on the methodology of the test a little... Atlanta is over 1000 feet over sea level and Marietta is higher still since it is more towards the mountains. Winter Haven is pretty much at sea level. So on the trip up you were headed up hill and on the trip back, downhill. Just picking on the test conditions...
Now, I have noticed that the AC is dropping my mileage... from 40ish combined to 36ish combined.
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