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  #1  
Old 09 March 2013, 05:02 PM
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Driver Debunking fuel-economy myths

Like all drivers, you want to save gas and do what’s right for your car. But along with the tried-and-true advice, there are some well-intentioned — if off-the-mark — tips that can lead you astray. Below are several common myths about fuel use and gas mileage, and the real stories behind them.

http://www.consumerreports.org/cro/m...yths/index.htm
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  #2  
Old 09 March 2013, 05:13 PM
RichardM RichardM is offline
 
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"A dirty air filter drops gas mileage"

CR says this isn't true. I disagree, it is true although maybe to a negligble point. Pulling the air through a dirty filter takes more energy that pulling it through a clean filter. That can be proven in the lab although apparently not on the road.
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  #3  
Old 09 March 2013, 09:08 PM
Alchemy Alchemy is offline
 
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Yeah, the filter thing seems a little odd.

Catalytic converter technology has required careful control of the fuel air mixture on practically every gasoline-fueled passenger vehicle in the US for the past 30 years. If the mixture was so badly controlled that a slight increase in filter pressure drop due to particulate buildup could continuously enrich the mixture enough to affect fuel economy, that would produce an absolutely massive increase in post-catalyst CO and hydrocarbons. Such a car would never pass emissions. I don't think anyone very familiar with engines would think this a serious concern for a light passenger vehicle built after (or perhaps even during) the 1980s.

The actual issue, as you state, is the pressure drop increasing the (effective) throttle loss. I'm curious how thick the filter cake on the "dirty" filter got.

Still, filter blockage only adds to the pumping loss already generated by the throttle, and it's entirely possible that, to a limit, a dirty filter simply offsets the throttle setting.
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  #4  
Old 10 March 2013, 12:55 AM
Meka Meka is offline
 
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I'd have to question the "Windows down vs. A/C" one as well, as I suspect that may vary depending on speed and the type of car. The Mythbusters, for example, found windows down to be more fuel efficient on an SUV at 45 mph, but air conditioning to be more efficient at 55 mph.
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  #5  
Old 11 March 2013, 07:30 PM
RocMills RocMills is offline
 
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Quote:
Originally Posted by Meka View Post
I'd have to question the "Windows down vs. A/C" one as well...
Definitely depends on the type of car being driven. My hybrid gets much worse MPGs if the A/C is cranked up.
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  #6  
Old 11 March 2013, 08:12 PM
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I would also expect a car that's designed to be very aerodynamic like a Prius to suffer more from the affects of wind resistance when the windows are rolled down, as well as the drain from running the AC.

I usually roll down the windows when I'm driving at low speeds, say less than 45 mph, and roll them up and turn on the AC on the highway. That's not just because of the fuel economy thing, but also because the wind noise just starts to become bothersome at higher speeds.
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  #7  
Old 11 March 2013, 11:27 PM
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DrRocket DrRocket is offline
 
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When it comes to AC use, YMMV. I keep a fuel mileage log for my car and I can positively say that when I use the AC in the summer, my mpg drops a little more than 1/2 mpg. I can also positively state that anyone riding at freeway speed in my car for more than 10 miles with the side windows down will be begging me to roll them up and hit the AC. The noise is really something, expecially if the rear windows are down. The owner's manual even warns about this.

The OEM tires are Goodyear Assurance Fuel Max low rolling resistance tires. They offer excellent wet and slippery/snowey weather traction. When needed, I'll replace them with the same exact tire. That won't be any time soon. After 60,000 miles, there's plenty of meat left on them.
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  #8  
Old 11 March 2013, 11:40 PM
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WildaBeast WildaBeast is offline
 
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Quote:
Originally Posted by DrRocket View Post
I keep a fuel mileage log for my car and I can positively say that when I use the AC in the summer, my mpg drops a little more than 1/2 mpg.
I kept reading that as "one-half mpg" and it took me a few minutes to realize you most likely meant "one to two mpg." I was about to ask you a) you you manage to measure your fuel economy so precicely, and b) how the drop from using the AC could be so small.

Even a 1-2 mpg penalty for running the AC doesn't seem all that big to me. Probably because I have a small car with a small engine the my mileage can drop by as much as 5 mpg when I use the AC. For pure highway driving as with a long road trip I can achieve close to 40 mpg with the AC off, but it drops to more like 35 mpg with the AC on.
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  #9  
Old 12 March 2013, 12:44 AM
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DrRocket DrRocket is offline
 
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You read that right the first time. A little over .5 mpg.

I've recorded mileage driven and gallons purchased every fillup since the day I bought this car. Since, the primary use of this car is for transportation to and from work, I drive the same amount of miles over the same roads every week so I can keep fairly accurate records for it. In the spring and fall, I don't use the AC. In the summer, I do. After I read this, I grabbed my log and confirmed the math. But I pretty much knew the figures anyway. In the coldest winter months, mileage drops about 2 mpg.

The best ever has been 46.3, the worst 31.8. Oh, when I put a K & N air filter in, it went up 1 mpg.

I haven't bothered testing with the windows down because of the noise, and I like AC.
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  #10  
Old 12 March 2013, 01:03 PM
overyonder overyonder is offline
 
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Quote:
Originally Posted by DrRocket View Post
when I put a K & N air filter in, it went up 1 mpg.
Probably due to a less restrictive air flow. That being said, K&N filters dust trapping abilities are often questioned, some people say they work wonderfully, some people say that they are not trapping all dust. YMMV. [Just one of many hearsay).

OY
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  #11  
Old 12 March 2013, 04:28 PM
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Ali Infree Ali Infree is offline
 
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CR has a bias about low rolling resistance to my mind. I was looking for new tires for my car and searching their site. The top-rated low rolling resistance tires were described as having much longer stopping distances and as not good on snowy/wet roads. I went with Fuzion tires on the front which have relatively low rolling resistance in a performance type tire. I think they work just fine in the wet and very well on what little snow we have had this winter. And I know that they wear well, based on using the tires before on a different car.
This is a little like when CR tests a high performance sports car and describes the ride as harsh. It isn't supposed to ride like a family sedan.
On the confession side, I have been using high test gas inspite of knowing that it doesn't make any real difference. The car seems to run a tad more quietly on higher octane.

Ali
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  #12  
Old 12 March 2013, 07:12 PM
RocMills RocMills is offline
 
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I have the advantage of rarely (almost never) needing to drive on the freeway... my usual trip is less than 10 miles and all on surface streets at speeds of 25-45 mph. This makes it easier for me to stay away from a/c use. When I put the windows down, it's usually no more than halfway. The few times I have run the a/c while driving around time, I have noticed something like a 5-10 mpg reduction in fuel economy.
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  #13  
Old 12 March 2013, 07:48 PM
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WildaBeast WildaBeast is offline
 
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Quote:
Originally Posted by DrRocket View Post
You read that right the first time. A little over .5 mpg.

I've recorded mileage driven and gallons purchased every fillup since the day I bought this car. Since, the primary use of this car is for transportation to and from work, I drive the same amount of miles over the same roads every week so I can keep fairly accurate records for it. In the spring and fall, I don't use the AC. In the summer, I do. After I read this, I grabbed my log and confirmed the math. But I pretty much knew the figures anyway. In the coldest winter months, mileage drops about 2 mpg.

The best ever has been 46.3, the worst 31.8. Oh, when I put a K & N air filter in, it went up 1 mpg.

I haven't bothered testing with the windows down because of the noise, and I like AC.
I actually keep a log like that, too, but I drive so little and my driving is so varied that any variation that small is more likely due to a change in my driving, not due to AC use. So like I said the only time I can get a really good comparison is on a long road trip. I might drive to the Bay Area and back during the summer with the AC running, fill up, and get 34 mpg, and make the same drive in the fall with the windows up and no AC and get 39 mpg.

I assume your car's probably a lot newer than mine. I wonder if now that the EPA test actually factors in AC usage auto companies have been motivated to improve the efficiency of their air conditioners. >>
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  #14  
Old 02 November 2014, 02:11 PM
urbanlegendfanatic urbanlegendfanatic is offline
 
 
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Here's the latest myth about saving on oil going around - I don't know if there's any truth to it and I'm not about to try it out.
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  #15  
Old 02 November 2014, 10:46 PM
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Roll eyes

If your car is powered by a hydrogen fuel cell, maybe. Otherwise, it's about as effective as dumping sand in your fuel tank.
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  #16  
Old 03 November 2014, 02:40 AM
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That and even if it did "aerate the fuel", I don't see how "operating on fuel vapor instead of liquid" would increase you mileage. I'm not sure if the car would even run. The fuel injectors require fuel at a certain pressure in order to operate. I don't think the pump would be capable of getting fuel vapors up to the right pressure and to the injectors. And on an old carbonated car it would be even less likely to work as the carburetor requires a supply of liquid fuel in the float chamber.
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  #17  
Old 03 November 2014, 04:39 AM
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If you add 7 cups of baking soda to your vehicle's fuel tank, the baking soda will clog the fuel line and your vehicle will be shot to s*it. But hey, maybe it will smell better. Who knows?

This has been a Public Service Announcement.
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  #18  
Old 03 November 2014, 11:30 AM
Singing in the Drizzle Singing in the Drizzle is offline
 
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It is more efferent to burn gas as vapor instead of liquid, but there are many problems with it. The first one is safely converting the gas to vapor fast, safely and delivery to the engine. It tends to burn a lot hotter so you need to have a much improved cooling system on the motor. Then you need an efferent delivery and mixing system designed to work with vapor instead of liquid. In short it can work but you will have to redesign your engine for it to work.

Just adding baking soda to a gas take does sound like it might be good for the environment because your car may never pollute the environment again with CO2. This do to engine no longer functioning.
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  #19  
Old 03 November 2014, 03:21 PM
Elkhound Elkhound is offline
 
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How about switching to a motor scooter? That'll save you money. (Yes, I know it isn't reasonable for people who have to haul kids or goods.)
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  #20  
Old 03 November 2014, 03:28 PM
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Quote:
Originally Posted by Singing in the Drizzle View Post
It tends to burn a lot hotter so you need to have a much improved cooling system on the motor.
Another problem with the higher combustion temperatures is that it will produce significantly more nitrogen oxides, a major urban air pollutant.
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