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How can I improve my gas mileage?

 
 
Reply Sat 28 Feb, 2009 09:25 pm
I'm wondering how many things actually affect gas mileage, besides the common things like keeping the car tuned up, etc. Does running the air conditioner really affect mileage? If you don't run the A/C and roll the windows down, does the drag resistance negate the savings? What is the difference in gas mileage (on average, since all cars' mileage varies) between driving 55 mph and 65 mph? What is the best speed on the highway to get the best mileage?
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Type: Question • Score: 5 • Views: 3,337 • Replies: 18
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roger
 
  2  
Reply Sat 28 Feb, 2009 10:04 pm
@CLTBenson,
Air conditioning has a significant effect on gas mileage. Rolling the window down doesn't. This defies conventional wisdom, but that's how it is.

The difference in mileage due to increases in speed occur because wind resistance increases as the square of the increase in mileage. If you've got a big, boxy thing like a Hummer or Element, it is a big deal, though I can't quantify the actual change in fuel consumption per mile. The best tip is to use speed control to avoid big changes in speed. Acceleration drinks gas like nothing else. If you don't have speed control, drive like you do, anyway.
parados
 
  1  
Reply Sat 28 Feb, 2009 10:35 pm
@roger,
Experts tend to disagree with you on the air conditioner roger.

It depends on the car and the speed.
TTH
 
  1  
Reply Sat 28 Feb, 2009 10:36 pm
@CLTBenson,
Keep your tires inflated properly. Underinflated tires roll more which causes you to gas up more.
0 Replies
 
GeneralTsao
 
  1  
Reply Sat 28 Feb, 2009 10:41 pm
If you can find it in your area, buy 100% gasoline. You will gain about 20% in mpg over 10% alcohol-contaminated gasoline.

I drive the same 800-mile route a lot, and get about 10 mpg when loaded, and going about 60-65 mph. When I switched to E10 to save that dime, my mpg dropped to about 8.

I have a friend who drives a Geo to work 15 miles every day. He says his mpg loss is more than 20% when burning E10, based on the miles he gets per tank.

The biggest gyp in the US right now is selling E10 fuel for 10 cents less per gallon than E-Zero (a fiscal savings of about 2.5-6% depending on your fuel price) but losing 20% in fuel mileage.

It is not the fuel *prices* that are causing record profits for the oil companies. It is the dilution of the fuel!

Oh, and it was mandated by the government. I wish the government would mandate a profit increase in my business!
0 Replies
 
roger
 
  1  
Reply Sat 28 Feb, 2009 10:58 pm
@parados,
I'm aware of that, parados. I don't remember whether it was AAA or Consumers' Reports that actually tested the thesis.
0 Replies
 
kuvasz
 
  1  
Reply Sun 1 Mar, 2009 01:19 am
drive downhill whenever possible.
roger
 
  1  
Reply Sun 1 Mar, 2009 01:36 am
@kuvasz,
Never thought of that one. How could I be so dumb?
jespah
 
  1  
Reply Sun 1 Mar, 2009 09:34 am
@roger,
Empty your trunk except of everything you need. I've known people who used their trunk as a mini garage, with beach chairs in it or whatever. If you don't need it, find another home for it. Carrying around the excess weight is a drag on mileage.
0 Replies
 
parados
 
  1  
Reply Sun 1 Mar, 2009 09:58 am
@CLTBenson,
One of the biggest things you can do is change your acceleration and braking habits.

Hypermilers will try to never come to a complete stop because, it uses fuel to idle and it uses fuel to get the vehicle moving again. They also suggest avoiding left turns.

You can achieve some fuel savings by turning the engine off at stop lights. Hybrids do this automatically.
0 Replies
 
ebrown p
 
  1  
Reply Sun 1 Mar, 2009 10:30 am
Drive less.
Carpool.

(These don't improve mileage exactly... but they have a far greater impact on your wallet and your environment then any of the other suggestions)
0 Replies
 
curtis73
 
  2  
Reply Mon 2 Mar, 2009 03:01 pm
Here is a list of some things that always work.

-Proper tire pressure
-skinnier tires. Those big fat meats might look nice and handle a little better, but they increase rolling resistance considerably
-smaller wheels. 24 inch "Rimz" look nice, but they add inertial resistance. That is only a factor during acceleration, but they can have a large affect on MPG
-lighter weight. Like someone said, take the mobile office out of the trunk
-put a block of wood under the accelerator. Seriously. If you can only open the throttles 60%, you're only ingesting 60% as much air, which means the engine is only injecting 60% as much fuel. That won't change highway cruise MPG, but the amount of fuel you use on those full-throttle blasts is quite significant. I put a spring under the pedal of my pickup for a long trip. That way I only used about 50% throttle before I hit the spring. If I needed it, I could still push hard and get up to 80% throttle, but it prevented excessive use of the right foot. Diesel Power Magazine did this test on a Cummins diesel in a Dodge pickup and went from 22 mpg to 28 mpg.
-Driving slower can help, but sometimes hurt. There is a huge dance of numbers going on; aerodynamics, rolling resistance and friction, gearing, engine design, how and where the transmission shifts etc. Many times people think that switching the axle gears to a higher ratio will save fuel, but it won't if it now requires more right foot to maintain speed. The same thing is true of driving slower. If you drop down to a speed where you are constantly needing to shift up and down, then you are lugging the engine. That is to say, the RPMs are below the torque peak, and it requires a lot more right foot to maintain speed. The best engine efficiency (not MPG, they're different) happens just below the torque peak. Engine efficiency means that of the fuel you are giving the engine, it is converting the most energy stored in the fuel into power at the wheels. If the engine is designed so that this peak engine efficiency occurs at the same time as your gearing puts you at the speed limit, you'll get best MPG from your combo. BUT... put that same engine and gearing in a pickup truck, and the aerodynamics will kill MPG. In that case, it might be wise to slow down a bit. The reduced engine efficiency might be offset by the reduction in wind drag.
-A/C uses more fuel than having the windows down. Period. Its documented a thousand times over. A/C systems can use as much as 10-15 hp to operate. Most cars on the average need about 40-60 hp to maintain highway speed. Kicking on the air might add 15% more load to the engine, meaning you'll have to use 15% more right foot to keep up. Rolling the windows down has an unpredictable effect on drag, in some cars it can even improve aerodynamic drag figures. People find that hard to believe because when the roll down the windows they are all of a sudden hit with all this torrential winds and noise and assume they have just added wind resistance. That noise and wind is still happening when the windows are up, its just that you can't hear it. Rolling down the windows changes surprisingly little.
-retune the computer to inject less fuel. Sounds weird, I know. Most vehicles come from the factory with a target A/F ratio of around 12.3:1, which is a little rich. They do this for three reasons. First, it helps with reliability by preventing detonation. Second, it keeps the combustion cooler which lowers NOx emissions. Third, it helps keep the catalyst screaming hot to help lower other emissions. Ever notice how aftermarket tuner chips claim "10 more HP and 10% better MPG?" That's because their primary function is leaning out the fuel mixture to more like 14:1. Still safe, still passes emissions, but uses less fuel.

There are more that I'm sure I'll think of later...
ebrown p
 
  1  
Reply Mon 2 Mar, 2009 03:27 pm
@curtis73,
Quote:
Kicking on the air might add 15% more load to the engine, meaning you'll have to use 15% more right foot to keep up.


Your argument about AC is just silly... from your unscientific statements about drag to your claim that it it is "documented" that lacks any link to documentation.

For the record, I have the exact same about of "right foot" when I use the AC as when I don't.
curtis73
 
  1  
Reply Tue 3 Mar, 2009 06:39 pm
@ebrown p,
Quote:
Your argument about AC is just silly... from your unscientific statements about drag to your claim that it it is "documented" that lacks any link to documentation.

For the record, I have the exact same about of "right foot" when I use the AC as when I don't


Just because I don't provide links to the documentation doesn't mean its not documented. Dataloggers in my own vehicles, research done by the EPA, CARB, and other organizations, even simple MPG calculations when you fill up the tank.

And then you follow it up with your own completely unscientific claim. Hyprocrites rule, dude. What did you do, just guess on your foot position? An AC compressor is a high-friction pump that produces up to 200 psi. How can that possibly not create drag on the engine?

The rest is simple math. If your car requires 60 HP to maintain highway speed, and your A/C takes 10 hp to operate, that means you MUST increase crankshaft output by 10 hp to maintain speed. Period. How do you expect to make more HP if you don't add more throttle input?

Why don't you back up some of your claims with something more than "exact same right foot." Hook up a datalogger, put it on cruise, monitor the TPS voltage, then kick on the air. I guarantee that your TVA increases. Making stupid claims that fly in the face of physics won't win you any points. On the other hand, maybe my PhD in engineering and years spent as a designer/engineer for three major auto manufacturers doesn't mean anything either.
roger
 
  1  
Reply Tue 3 Mar, 2009 06:58 pm
@curtis73,
What you do is drive a 5 speed manual, with a 4 cyl engine in town. With A/C on, you'll always wonder why you are driving in one gear lower than normal.
0 Replies
 
ebrown p
 
  1  
Reply Tue 3 Mar, 2009 07:57 pm
@curtis73,
I think you are full of it, Curtis...

If you could post a link to the EPA research that backs up you claim it would help me take you a bit more seriously, but I don't really care so it might not be worth the time for either one of us.

I made no scientific claim... in any discussion of how much "right foot" I have is too vague a term to be useful (unless the answer is exactly 1.0 ... which was the humorous point I was trying to make).

I have never disputed that AC puts a load on the engine (or reduces mileage). The brash claim YOU are making is that is greater than the drag caused by open windows in any conditions with any car.

This is quite a claim for you to make. All I am asking for is for you to back this up with evidence (i.e. sources) which you claim are so easy to find.

"Reasearch done by the EPA" would be a good start....
gungasnake
 
  1  
Reply Tue 3 Mar, 2009 08:20 pm
@CLTBenson,
Some of the boxy looking American cars made in the 50s - 70s actually got best mileage around 55 and you'd also note that five speed transmissions were rare then.

More modern cars generally get best mileage at some point at which valve timing has become efficient in top gear and before air resistance starts to dominate the picture.

For many cars that will be more like 65 - 70 than 55 and you even had cars like that back in the previous age of cars. The EPA in the 70s used to rate the Alfa Romeos around 22 - 23 highway meaning that they were testing at 55 while it was common knowledge that an Alfa being driven at 70 would get more like 30 - 33. That was with the two side-draft Webbers before everybody went to fuel injection.

0 Replies
 
curtis73
 
  1  
Reply Thu 5 Mar, 2009 01:13 pm
@ebrown p,
Quote:
The brash claim YOU are making is that is greater than the drag caused by open windows in any conditions with any car.


You're right... brash claim. I'm sure there might be some obscure vehicle that makes more drag with the windows down than the A/C, but finding it would be difficult, and in the perview of this discussion its very safe to say that there are no examples we can discuss. Let me phrase it this way; in any common, production vehicle produced for retail sale since about 1950, there are no vehicles that use more fuel with the windows down than with the A/C running.

One of the EPA examples tested was a car that I used to own. I had a 73 Impala Station wagon. The EPA actually tested a 72 Caprice wagon, but same thing. With the side windows down and the rear clamshell glass open, the car significantly LOST drag, like 1-2%. I remember hearing (second-hand info from that terrible "mythbusters" show) that at most, putting down the windows might increase drag by a maximum of 1-2%.

That is why I made such a brash statement. If A/C is responsible for adding 5-15% additional drag, and even the worst case scenario of windows being down can only practically cause 1-2%, its pretty safe to assume (with obvious exceptions) that AC will always use more than the windows being down.

And you're also right... I'm not going to sift through 45 years of archived EPA documents, most of which aren't online, just to prove my point. Smile

0 Replies
 
hamburger
 
  1  
Reply Thu 5 Mar, 2009 01:26 pm
@CLTBenson,
IMPROVE YOUR GAS MILEAGE :

http://www.fueleconomy.gov/FEG/drive.shtml

speed and fuel consumption :


http://www.fueleconomy.gov/FEG/images/speedVsMpg3.gif

0 Replies
 
 

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