ddcha

I used to get ~350miles on a full tank or 24 gallons.
The last two tanks however, I got ~315miles. I know running the A/C will guzzle more gasoline, but this difference seems a bit much.

I know both numbers are terrible, but this is mostly local driving and waiting on the car line to pick the kids up, etc.

Please share some insight!

blittle

Have a 2011 GL450..... do not notice much if any difference in mpg between cold weather driving and AC season! AC used to really put a load on less than adequate engines.... however, I think better technology on the part of the compressors and better engine technology has reduced the impact on mpg somewhat.

troymir350

You "lost" approx 35 miles over 24 gallons. That is a little less than 1.5 miles per gallon. If you are city driving and/or idling most of this time that is definitely within the realm of possibility. Although that is pushing 10% less than the city mileage rating if you do get around the stated city mileage rating of 15 ish mpg.

The compressor is more efficient, but it is still driven off the engine and takes energy. The AC load on vehicles and the energy that the AC systems consume is a hot topic currently, as is the change of the refrigerant that is inside the system. Greenhouse gases and the related emissions from vehicles running AC contribute to the ozone reduction (not Al Gore's movie, real science). And I stated CONTRIBUTING!

Wait until we're all forced to use start/stop systems (see future Mercedes vehicle plans!) and it's summer in the city! Then you'll see how nice that AC is and how easy it is to pay that mileage penalty.

giffie

I am still confused on how the AC works...
Question: Does the compressor consume the same 'energy' when the AC is operated at 74F v/s 60F?
I thought that the amount of energy consumed is the same, but the compressor turns 'off' when the set temperature is reached. It turns 'on' when the temperature rises a deg above the set temperature. This is wrong, I guess?

troymir350

I'm sure there are people on here way smarter than me, but in general you are correct that the compressor does shut off at some points. When the compressor is "off" it is decoupled from the drive shaft and does not take away from the engine power.

In general (to your example), if you use auto and set to a temperature the AC evaperator and heater core work in tandem with duct doors in the vehicle to vary the amount of cold and hot air so that your set point is reached. The AC system puts out a fixed temperature and the heater puts out a fixed temperature. It's only a series of valves (heater) and doors (AC) that blend the air together until a temp sensor in the vehicle tells the HVAC controller that the temperture set point has been reached. Then the compressor will stop until called for again.

I personally don't use the auto mode, as I prefer to choose when the compressor runs. In manual mode you can dictate the compressor on/off time.

ddcha

10% lower fuel efficiency from the AC being on a lot seems excessive to me. The other culprit I thought of was tire pressure. I checked today and the fronts were a little low so I added some air. I guess I'll have to see how many miles I get out of this tank. Just refilled.

Our driving patterns are very predictable. When we're not on vacation, there are always the same pick-ups, drop-offs, and drives to the mall, church, etc. All mostly local driving. We've ranged in the spring about 340-360mi for a tank. Now that AC season hit, we're at 310-320.

pteam

If this is true, then the "auto" mode could futher lower MPG because its running the heater also. If you just set it on 60 degrees, from what you say it sounds like the heater core wont run at all and would consume less power thus equaling more power.

Personally I set it on 60 degrees not on auto, and turn the fan speed down once its starts to get chilly in there. When I get really cold I'll turn the AC button off but still have the air recirculating, and then turn the AC button back on once I start to get warm again.

Also if nobody is riding in the back of the vehicle turn the rear air off, this will definately help with MPG because you wont have the fans and additional systems running at all back there.

troymir350

If this is true, then the "auto" mode could futher lower MPG because its running the heater also. If you just set it on 60 degrees, from what you say it sounds like the heater core wont run at all and would consume less power thus equaling more power.

You are correct in that turning off rear air and such will keep more flowing to the front. But, the heater core doesn't consume power in the sense that it is taking the heat generated by the engine that is transferred to the coolant. The heater loop goes from the engine (heat transferred to the coolant), and runs through the heater core (to heat the cabin unless the heater core is not directed to the cabin or is shut off by a valve), and then to the radiator where the heat rejection takes the heat out of the coolant so that the coolant can go back to the engine to remove more heat, as a cooler fluid able to accept more heat.

blittle

In short.... IF you keep your system on "auto" winter and summer, you will not notice much difference in mileage season to season. If, on the other hand, you turn the system OFF..... you WILL see an increase in mpg albeit probably not that great. Mine is on "auto" all of the time..... I tend to get around 14 at home and 19 on long interstate runs.

YYZ-E55

The climate control maintains the interior temperature on start-stop vehicles when stopped. It is actually a fantastic system on standard Euro Benz models; I wish my GL had it.

kkwmjw1

Ok, I'm gonna be the jerk in the room and say, who cares. We plunked down 80000 dollars let's say for a 2.5 ton gas guzzling behemoth that the tree buggers hate and OPEC loves and we expect to get good mileage? If it's 90 degrees outside and sitting in the drivers seat with the air at 68 and the vented seats cooling my back I don't care what the mileage is. I didn't buy my 550 for the gas mileage, if that were the case, I would have bought a Prius, ( dear god what a mistake that would be). This reminds me of the people who complain about 4 dollar gas and then go in the store and pick up a pint of bottled water for a 1.50. 12.00 dollars a gallon for water but we don't complain about that. When's the last time you heard any one say " Did you see the price of water today? Damn it went up another 10 cents" Not trying to offend any one just having a little fun. I sometimes think we have are priorities mixed up.

blittle

Well said...ah, except for the Prius thing.... I DID buy that one for what it is and I have the GL for what IT is! Best of both!!

LEOSOPHIE

I am the second jerk.

kkwmjw1

Sorry about the Prius joke

mdadgar

I think the difference is that people tend not to spend $100/week on bottled water so they can commute to work.

- Mark, lives in CA where gas is extortionately expensive so bought a diesel

kkwmjw1

Probably true, but if we took all the little vices we have and never complain about ,I'll bet we get to a 100/ week and more in a hurry. How about the 4 dollar Starbucks or my personal vice, a few beers or cocktails at 4 to 5 dollars apiece. That comes out to probably 30 dollars a gallon. Like I said before, not trying to offend anyone, just trying to put some of this in perspective. Bought my GL550 because it looks bad ***! End of story.

AsianML

I don't think this thread and this concern was brought up to highlight the difference in gas mileage and the resulting cost differences. More or less to figure out if the drop in efficiency is at an acceptable level and comparable to that of other GLs - and if not, see if there's something wrong and if something can be done to correct it.

It is not unreasonable at all look further into something that doesn't look or seem right.

ddcha

You make good points. I'm not complaining about bad fuel economy in general. Heck, I have 4 kids in tow, so on a per capita basis, the fuel consumption is damn good. It's the sudden decline that has me wondering. If you found yourself filling up the car every 6 days a week instead of the usual 7, wouldn't that make you wonder if something was wrong?

ddcha

The ***** has spoken the truth

blittle

Enjoy your vehicles and the knowledge available in forums such as this one...... live, learn, and laugh! Generally in that order

mdadgar

Hey, we're on the same page here. But people are more sensitive about the things they have to buy (gas) than the things they want to buy (internet por ... Starbucks lattes).

- Mark

alx

Some good, some bad answers here...

5- 10% loss when running ac is about right.

Parasitic drag is more pronounced at higher rpms... So less effect on diesel, more on gasoline engines.

Compressor is also decoupled at wot.

Heating does not use more energy directly. The only way it will waste gas is on a cold day when engine takes longer to reach op temp as the heater is suckingn heat to blow it into the cabin. Colder engines use more gas.

Best most efficient way to run climate control is to run on auto, but turn on/ off ac manually as needed. Generally if outside temp is less than 2 degrees cooler than requested cabin temp- turn off ac. In other words if cabin is set to 70 and outside is 68 or lower no ac is needed.

When ac is on compressor is cycled as needed. Generally when temp rises a degree or so the compressor is turned on and when the temp drops a degree it is turned off. Thus the lower the target temp the moreoften the compressor is turned on.

ddcha

I found the culprit. I spoke the gas station manager (I fill up at a local Shell station on V-Power). He said that they recently had similar inquiries. His conclusion is that Shell has made some changes recently to its ethanol blend. V-power used to be ethanol free, but they have been slowly increasing the ethanol content. He thinks a higher ethanol blend was recently distributed in NJ.

mdadgar

Lucky you. We've had 6% ethanol blend (for "oxygenation purposes") here in CA for years. It sucks.

Thanks, Big Corn Lobby!

- Mark

AsianML

I would kill for 6%! Most stations in Michigan have been pushing 10%.