Actually, one should read the details about tire inflation, because I don't think it's accurate to say they concluded one shouldn't do anything about it:

http://www.edmunds.com/ownership/driving/articles/106842/article.html#test5

I've always wanted to know about the comparison between using AC and having the windows open. The analysis make sense. According to their suggestion, now I don't have to sweat so much.

Mythbusters episode 22. Fully demonstrated that until you surpass about 52 mph, windows down or AC is the same.

I sure hope they were not suggesting not to do anything about tire pressure!

Note on AC, most AC in cars continously leak their cooling medium, thereby contributing to the greenhouse effect, even when not in use.

How about buying the right kind of vehicle? Driving to work every day in your Dodge Ram Quad Cab doesn't make any sense.

When I was commuting 40 miles a day, I was able to coax 37mpg out of my 1999 Mazda Protege by following these tips.

• I accelerated slowly
  
• I found the quickest routes off of the surface streets and onto the highways
  
• I pulsed my A/C - Ran it full blast until I got cool and then turned it off
  
• Set my cruise for 60 mph, as slow as I could go without breaking the law. More than 5mph under the speed limit is illegal in some places
  
• Coasted down the couple hills I could while maintaining highway speeds
  
• I learned the route and knew the light cycles. That way I could turn the car off at long lights. This might be illegal. But how's anyone going to know? Just remember to pay attention at stop lights.

Of course, my mileage now is infinite, because I moved to within walking distance of my job. And as any math student will tell you, any number divided by zero (as in x miles/0 gallons) is infinity.

I saw that Mythbusters too. I'm keeping my AC on! I also saw on there that keeping your pickup truck's gate up is actually better than down, because it creates a circular flow in the bed.

The are not advocating for one to ignore tire pressure. They are stating that mileage results are negligible.

A quote from the article:
"Check your tire pressure often but don't expect a big savings."

Most cars generate low frequency, obnoxious sounding "beats" at sustained speed over 45, if windows are left partially open. This overwhelms music, and the turbulence created by fully opening them far enough to stop the beats generates complaints about hair blowing. More than one reason to leave the windows up when AC is most needed.

They are stating that mileage results are negligible.

And if you look at their comments, you'll see they overinflated the tires part of the time, and the lowest they went was 8 pounds under recommended pressure.

I see tires WAY below that every day, and it's just simple physics that that will reduce mileage.

Yep, lift gate up on that pickup that shouldn't be driving your sorry butt to the grocery store anyway. ;)

Tire pressure can make a sizable difference in mileage. People driving around 10psi lower than spec are wasting a good amount, though I have no concrete numbers. Low PSI also results in uneven treadwear, resulting in short-lived tires. Low PSI is also a safety hazard, decreasing handling and slowing down steering response.

The real trick is to run a few PSI higher than spec. Some street racers do this for increased handling. Make sure not to exceed the max PSI of the tire, taking into account very hot days and such. I run my tires 6 PSI over spec for my model. Slightly harder ride, but better handling and a few percent better MPG. Technically tires may wear a bit faster, but I have always exceeded my tire's rated life doing this, so it might just be more about "how" you drive.

Joseph's right. In the real world, you can't throw a tenis ball without hitting someone that has underinflated tires, way below 8 pounds under recommended pressure.

Pieter if what you say about that AC in cars is true then how come my 6 year old Mustang with 79,000 Miles on it has AC that still blows cold. If the AC in a car leaked then it would not work for very long. You are thinking of the "bad old days" about the pre 1990's when car quality wasn't very good. AC is a sealed system and doesn't work if it leaks.

If you feel this is incorrect please sight the source of your information.

I've decided that the hit to fuel efficiency based on running the AC or not is dependent on the power and size of your engine and your driving style. It's definitely not a one-size fits all efficiency strategy. Based on my car and my driving style, running the AC definitely kills my MPG. I attribute it to:

a) My car's small engine - I think if you have a bigger engine, the power consumed by running the compressor as a percentage of total power available is notable. If running the AC zaps about 15 HP, and your car has a 250 HP engine, you're barely going to feel the AC turn on. But if you have a smaller car with a 100 HP engine, you will definitely feel the loss. To those of you with smaller cars, notice how much power you lose when the AC is on.

b) My driving style - My commute to work necessitates driving the highway where it's not uncommon to oscillate between 20 MPH and 60 MPH. Because I need to call on my engine's ability to accelerate so much during the commute, running with the AC on only exacerbates any normal loss from speeding up and slowing down. To add to this, I usually drive in 5th gear at 35 MPH+. It's really iffy to drive between 1000-2000 RPM with AC on because you never have reserve power to accelerate as needed.

I never drive fast enough with the windows open to have the car's drag be more of a drain on the efficiency than the AC running. Maybe on long trips in the dead of summer with an open road ahead of me, but not for 99% of my travels.

Edmunds is all well and good - but you really need to test all these tips for yourself. What do you have to lose?

As I mentioned in my blog, the best thing you can do is turn off the aircon.

Petrol is 10% more costly this year but we've decreased our petrol expenditure by almost 33%.

This is great to hear. In my 10 year old Toyota Corolla I get 40-45mpg highway by staying at 60-65mph speeds. However, I get a lot of road raging from highway drivers who like going 70-75 minimum speed.

Someone oughta make a bumper sticker: "65 saves gas / slow down" - I'd wear it!

I'm sorry to have to bust bubbles, but driving at a slower velocity doesn't save you very much gas at all vs a faster velocity, and within a given range it doesn't save you any. Maintaining a constant velocity only requires enough fuel to overcome friction and drag... mostly the drag if you've you've maintained your car. I've run numbers on my car, and the difference between 75 mph and 55 mph is approximately .23 mpg. Saved. That is to say, I get better mileage at 75 than at 55. (It goes downhill fast after about 77 mph.) Common sense says they didn't consult your cars EPA rating when they set the speed limits.

The overwhelming majority of fuel used up is in acceleration. If you want to save fuel, then accelerate as easily and smoothly as possible, and try to keep enough room between you and other cars that you don't have to do a lot of braking because that's energy wasted. After that, maintain a constant velocity. On some cars, you're better off doing this with your foot because the cruise control tends to lope - accelerate too fast, coast too long.

As to tires, proper inflation has more to do with safety than fuel economy. Improper inflation can cause dangerously poor handling, and wear your tires out more quickly. Where wheels relate to fuel economy is in their rotational inertia, which is a function of their overall mass and diameter. That's why you see small skinny tires on econoboxes.

Relationship of velocity to fuel efficiency is a function of aerodynamics. A boxy Scion xB or Ford 150 is going to throw a turbulent wake of air 75 compared to the Toyota M2 (a sporty model). If all vehicles were similar in shape, added speed, reached through slow acceleration, would make little difference in relative efficiency. The change from linear to turbulent air streams will, however, be at a different speed per each design/shape. As someone else said, it's not a one size fits all situation.

Nice one! At least, it has been proven that you still could have great fuel mileage without a hybrid car. Question is, who's going to tell if the crash ratings are true? Like this article on Ford's topping IIHS' crash ratings? I, for one, am not going to do the testing myself.

“This is great to hear. In my 10 year old Toyota Corolla I get 40-45mpg highway by staying at 60-65mph speeds. However, I get a lot of road raging from highway drivers who like going 70-75 minimum speed. Someone oughta make a bumper sticker: "65 saves gas / slow down" - I'd wear it! Posted by: ProgGrrl"

Be careful with this. Speed limits are notoriously low in this country. The accepted method of determining a speed limit for a given stretch of road is:

When a stretch of road is opened, no speed limit is posted. Monitors determine the speed at which 80% of drivers drive at or below. That is then the posted limit.

This country gave up on that back in the 70's gas crisis, then never went back due to the immense revenue derived from speeding tickets.

So if you want to drive "slower", STAY IN THE RIGHT LANE. On single lane roads, don't drive below the posted limit. Doing so is a very passive aggressive blow to everyone behind you as the posted limit is most probably too low anyway. When a passing zone comes along, try to assist those behind you in passing: slow up a bit if the oncoming is clear so they have a big window to pass you. Bottom line, BE CONSIDERATE of other drivers ABOVE your desire to save a few ounces of gas.

Our car (Renault Scenic, 1999) gave figures for efficiency with windows open, sunroof open, and AC on. The sunroof used least power, the AC used a bit more, and opening the (front) windows used vastly more power than the AC.
But that's on a nice aerodynamic european car. I imagine the effect will be less pronounced on boxy US cars.

Aerodynamics in fuel economy is WAY oversold. The drag coefficient for a typical light truck 0.5. The drag coefficient for a typical economy car it is 0.4-0.5. Drag is a LINEAR relationship at subsonic speeds. So whether you're driving a Toyota Prius (0.26) or a Toyota Tacoma (.44), drag actually imparts relatively little force to the car at any speed. The internal friction of the drivetrain is by far a bigger factor than drag in terms of forces that need to be applied to maintain a constant velocity. Think about it this way: take a Honda Civic with an automatic, and a Honda Element with a stick. Accelerate them both to 45 mph. If the driver of the Civic (CD=0.36) takes their foot off the pedal, and the driver of the Element (CD=0.57, which is the same as the much loved Hummer H2) simply puts the clutch in, the Element will coast a lot further than the Civic. (The driver of the civic can even pop the car into neutral, and the friction + viscosity forces of those few extra gears soaked in tranny fluid will cause it to coast to a stop in less distance than it's fat un-aerodynamic Element brethren.)

Acceleration is where most of the fuel in a car is consumed; aerodynamic forces is the least concern.

Actually, Andrew, the Element would go farther in big part because of inertia; but also, because of it's higher mass, it took more energy to accelerate to that speed in the first place.

Aerodynamic is a pretty important factor at highway speeds. Nobody said it was the most important one (drivetrain friction, friction from tires on the road, weight, etc), but it is certain non-negligible. It's different at non-highway speeds, but seeing how two very different vehicles coast is not that helpful.

A better test would be to have two vehicle with the exact same drivetrain/weight/etc, but have one that is aerodynamic and the other not, and drive them on the highway for an extended period of time. Now that would show us the difference...

Some people report reduced mileage from opening windows or sunroofs of the highway, so it's hard for me to believe that a SUV isn't that much worse than a Prius when aerodynamics is concerned...

Just put your hand out of the window on the highway and see how much force the air exerts over such a small surface.. Now image that over the whole front end of a car... It all adds up.

"At highway speeds, where fuel economy is best, more than 50 percent of engine power goes to overcoming aerodynamic drag."

http://autos.msn.com/advice/CRArt.aspx?contentid=4023460

MGR: You got me on the inertia point; I chose the Element and Civic to demonstrate the point about drag vs mechanical forces, and I probably oversimplified in the process. Let's toss about four people into the Civic to even out the 650 lb difference between the two and I will happily bet you a decent bottle of scotch that the same result will hold true.

The equation for drag force is F=.5pv^2ACd where p (really it should be rho) is density (1.225 kg/m^3), v is velocity, A is the forward surface area of the vehicle, and Cd is the coefficent of drag. For a 2001 Honda Civic EX, Cd is 0.36 and A is 20 [m^2]. For everyone's favorite waste of sheet metal, the H2, Cd=0.57 and A=46 [m^2]. At 70 mph (112 kmh, or 403 m/s), the total aerodynamic force on the hummer is 23x10^5 [N], which is a tad over three times that of the Civic. Now consider that to accelerate to that speed, the H2 needs 31x10^7 [N], or 100 times the amount of force. Likewise, the Civic needs 13x10^7 [N]. Again, this is an oversimplified case, but it's patently obvious that the big consumption comes not in maintaining a constant velocity (even with aerodynamic forces), but in getting to a given velocity. It takes several miles to match the fuel consumption of maintaining a constant velocity with the consumption of even a modest rate of acceleration.

I chose the H2 in the previous example because the ungodly surface area they have is more than three times that of the Civic (resulting in a Cd*A of 26.3 vs 7.34), but results in just under three times the amount of drag forces. Consider our Civic, and compare that to a 2004 Dodge Durango at 0.39, and the fact of the matter is that drag coefficients for most economy cars aren't actually much better than most SUVs. Factor in surface area and a tad more of a gap shows up: Cd*A for the Honda Civic is 7.34 while for the Dodge Durango is 10.65. (The H2 is 26.3!) What makes SUVs so much worse in terms of overall fuel economy is they are more massive, requiring more energy to accelerate. They're also rear wheel driven in most cases, which adds even more mechanical losses. (Economy cars are almost always front wheel drive, which eliminates an axle with it's moment of inertia, and a differential with it's complex gearset from the mix.) Again, I cannot emphasize enough that the biggest fuel cost at highway speeds isn't aerodynamics, it's mechanical losses.

Per your "hand out the window" argument, the Cd of a flat surface such as your hand is about 2. Turn the back of your hand into the wind, and cup your hand ever so slightly, and suddently the Cd drops to about 1.2. Make a fist and your Cd will come down to about 0.6. If you've spent any amount of time with your hand out the window, then you can pretty well imagine these changes in force and how even slight rounding of surfaces can yield large gains in efficiency.

I can simplify an areodynamics argument without all the fancy math.

Truckers.

Years ago in the 70's cab-over trucks were the rule because the tractor was shorter. Some states had rules about how long the tractor-trailer could be so shorter tractor, longer trailer, more cargo, more money.

Now none of the over the road trucks you see are cab over design and most trucks look like they spent some time in a wind tunnel. The tractors are longer which might require a shorter trailer in some states but it's made up on the fuel savings.

Note: many states relaxed the tractor-trailer length rules as well.

Actually Tim, cab over trucks are still very popular. The design tends to be used for regional transfer trucks. I don't think Mitsubishi Fuso makes anything that isn't cab over.

For long haul truckers, their disappearance had little to do with fuel economy. The first thing that happenned is the standardization of shipping containers in the late 70's to speed up loading and unloading operations at ports; this led to a lot of empty space on oversized 18 wheelers. The second is a change in OSHA rules which mandated that drivers actually sleep after a certain amount of time. This led to the rise of sleeper cabs which pushed the engine forward anyway. There are about half a dozen other lesser reasons, such as maintenance issues, safety, and engine size, but those top two were the bale that broke the camels back.

Incidentally, the Cd of a cab over truck is only slighly larger than a bottlenose from 'back in the day', at 0.68 and 0.67 respectively. Today, most trucks actually have a Cd near that of a passenger car, with the latest Volvo's sporting Cd's between .48 and .56. That amounts to huge fuel savings when averaged over millions of miles.

Incidentally, I reread my comments, and I may have misled some people on my intent. I don't want to leave the impression that aerodynamics don't matter. We're simply comparing oranges to oranges here. At highway speeds, the aerodynamic forces on a car don't require that much energy to overcome. The bulk of fuel consumption in driving comes from accelerating cars to speed. If you measure fuel economy on a car at 65 and 85, you may not even see a difference for hundreds of miles. Most people will gladly burn the extra gallon on a 250 mile drive to get where they're going 20 minutes faster.

i have to question the math here. first off, 112 kmh is not 403 m/s, thats off by a factor of 10 or so. secondly, 20 m^2 is roughly 200 sqf, and there is just no way a civic would have this large surface area. where did you find that number? third, while im not familiar with the formula F=.5pv^2ACd, it does, correctly, point out that the force, and thus the gas consumption required to keep constant speed, is not linear to velocity, it is exponential. i believe this contradict a statement that you did in an earlier post, saying that the difference speed does on gas consumption is neglible. my experience is that at highway speed, the difference is far from neglible and, as your formula points out, the difference between 70 and 90 is far bigger than the difference between 50 and 70. finally, what do you mean by saying the cars "need" a certain force to accelerate to some given speed? I dont see where you got those numbers from, but more importantly they really dont make much sense to me as the force required to accelerate an object to a target velocity depends on how long time you are willing to wait to reach target velocity.

finally, on another account. as an old competitive biker, my experience is that proper tire inflatement is paramount for results in the tracks. using low on air tires would easily bump my times 20%. why would it make less difference for cars?

i did some more processing on the math and it just doesnt add up for me:

you say it takes about 23*10^5 N to drive a hummer at 70 mph. 70 mph is rougly 30 m/s

now, w=fs & p=w/t gives p=fv

with v=30 and f=23*10^5, we get p roughly equal to 700 * 10^5. that is 70*10^3 kW, or ca 10000 horsepowers. i know those hummers have way to big engines but they are not that powerful, right?