Tire pressure and mileage

I noticed my '97 Outback was getting pretty lousy mileage (16 mpg), and called my mechanic. He said to call him back at the end of the week and
setup an appointment. Meanwhile, I decided to check the tire pressures, which were about 3 to 5 lbs down. I pumped them up to 30 all around and my mileage went up to 24 mpg.
Does this sound plausible?
Sheldon
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Sheldon wrote:

Hmm, Didn't know that? I always keep pressure little higher than it's called for. Low pressure not only affect your mileage, it can destroy tire due to over heating when driving on turn pike. If you raise the pressure too much it can negatively affect handling and ride. Also when you calculate mileage you do it over time after several fill up. Not just with one tank of gas. This is basic stuff.
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Did this over two tanks after filling the tires. The bad mileage was tank after tank after tank.
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Sheldon wrote:

Hi, Also check the pressure when tire is cold. I fill my tires with Nitrogen gas from Costco tire shop. Nitrogen is more stable. If pressure is too low or high yo'll know it when you drive.
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My Subaru dealer was filling tires on all new cars with nitrogen and adding $200 to the total cost. After one year they discontinued this. I was told they found it added negligible benefits, unlike when it is used in race cars, so they will use up their nitrogen supply during repairs only.
I suspect they stopped because they were getting too many complaints about the added cost.
BoB
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Even our county police have been running nitrogen for highway patrol. May give you some benefit but probably not worth the cost. Now, helium is the way to go if you really want to reduce unsprung weight.
Seriously, pumping your tires up will not only improve mileage, but handling as well. When I used to autocross a Corvair I'd run 50 psi rear and 30 psi front, with much wider tires on the rear as well due to the extreme weight imbalance. For a street Subaru 35 psi, or whatever the tire manufacturer's maximum is, should optimize gas mileage, tire life and handling. And check that air filter. I belive the Subaru interval is usually 20,000 miles, but If you've got more than 10,000 and it's underperforming, it's a good place to start with a small investment.
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suburboturbo wrote:

Hmmm, I always buy tires from Costco. Nitrogen is used and rotation, repair all included for the life of tires. They have good selection of Michelin, Good Year brand. If it is odd size tires, they can special order them. Price is very reasonable.
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Even our county police have been running nitrogen for highway patrol. May give you some benefit but probably not worth the cost. Now, helium is the way to go if you really want to reduce unsprung weight.
Seriously, pumping your tires up will not only improve mileage, but handling as well. When I used to autocross a Corvair I'd run 50 psi rear and 30 psi front, with much wider tires on the rear as well due to the extreme weight imbalance. For a street Subaru 35 psi, or whatever the tire manufacturer's maximum is, should optimize gas mileage, tire life and handling. And check that air filter. I belive the Subaru interval is usually 20,000 miles, but If you've got more than 10,000 and it's underperforming, it's a good place to start with a small investment.

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$200 freakin dollars? Geeze. Ridiculous.
-- Todd H. 2001 Legacy Outback Wagon, 2.5L H-4 Chicago, Illinois USA
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writes:

An entire cylinder of helium is only $100. Don't know about nitrogen, probably cheaper.
-John O
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No nitrogen, but always check pressures when cold and the sun isn't heating up one side of the car.
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Yes.
--
John Rethorst
jrethorst at post dot com
  Click to see the full signature.
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http://www.google.com/search?q=%2B "low+tire+pressure"+%2Bmileage
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I say no. 16 mpg is far below what you would expect. The low tire pressure let's the effective tire diameter shrink a bit, but not by that much. You should be getting 22-24 mpg even in traffic.
From my recent experience with this problem, I'd say try changing the air filter first.
Ben
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pressure let's the effective tire diameter shrink a bit, but not by that much. You should be getting 22-24 mpg even in traffic.

air filter first.

A car with low tire pressure probably has a bunch of other places it can be tweaked to get better milage. (Owner obviously not paying attention.)
Note however, most of the wasted energy is not just diameter reduction, it's friction bending and unbending the tire every revolution (thus the tire heating). Likewise, the car roll resistance is going to be different which might lead the driver to make huge changes in how they accellerate; keeping the foot on the gas all the way to braking point, and then braking, rather than coasting and NOT braking much.
16 mpg is a lot down, but from the standard 24 mpg I can go up a bit, or would be down to 20 or so just with 5 or 6 pounds of air difference.
Yes, check other stuff, but a change in MPG of 8 isn't that far out of reasonable for that simple change.
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Not to me. 8 mpg is way too much for 3-5 lbs less. I'd expect something in the 1-3 mpg range.
DK
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Sheldon wrote:

Hi,
Yes, it does.
From personal experience, I've found different tires produce quite different mileage characteristics depending on pressures. Some are pretty "pressure tolerant" meaning mileage won't suffer much when one drops a couple of pounds or more. Others REALLY start to show the difference. (Same issues w/ handling!) As many have suggested, I, too, run 1-3 lbs over the door sticker w/ most of my tires.
I check them about every two weeks, and keep 'em filled w/ a 78% nitrogen mixture. It cost me $200 for a pretty good compressor set up to use that mix... but you can get one for considerably less. ;)
Get a good gauge, too. Some like digitals. I've got a nice one and it's ok, but I prefer the real Accu-Gauge analogs (there are plenty of cheap knock-offs but I'd avoid 'em.) Pretty bullet-proof and they never need a battery. Get the style w/ the built in "bleeder" and you can adjust down to 1/4 lb increments on the large dial models if you wish (my digital's only good to 1/2 lb.) Local availability is spotty, so you may want to go online. I think the name of the place where I ordered my last ones was called "getagauge.com" or something really close--Google's your friend.
Pencil gauges aren't on my radar screen anymore: we used to get really good ones when I worked in service stations back in the late-'60s/early-'70s, but everything I've seen recently is cheapo, cheapo and not all that accurate.
Rick
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On Fri, 28 Mar 2008 20:37:49 -0800, Rick Courtright

PepBoys sells this brand, and it's pretty good. The one I bought recently developed a leak and wasn't accurate anymore, so I sent it back to the factory with a $3 check for S&H, and they fixed it and sent it back. The built-in bleeder button is very handy for double checking accuracy. I'm really waiting to see how durable mine will be before I sing further praises. I'm still mad at myself for breaking the Brookstone dial gauge I had for 15 years. That thing was GREAT.
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Rick Courtright wrote:

LOL'ing at 78% nitrogen mix!
hahahahaha!
Carl
--
to reply, change ( .not) to ( .net)

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Carl 1 Lucky Texan wrote:

Stuff I google up says less pressure loss using pure N2: http://www.tires-inc.com/shop/nitrogen.htm
I was taught that diffusion rates are normally inversely molecular weight dependent and with N2 at 14 and O2 at 16, the N2 tire should deflate faster. I suspect it doesn't because of osmotic pressure - the pressure of O2 trying to get in to dilute the N2.
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