Tire pressure and mileage

[Sheldon]

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

 

[Tony Hwang]

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

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.

 

[John Rethorst]

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

Yes.

 

[VanguardLH]

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?

http://www.google.com/search?q=+"low+tire+pressure"+%2Bmileage

 

[Uncle Ben]

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 andmy
mileage went up to 24 mpg.

Does this sound plausible?

Sheldon

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

 

[.._..]

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

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.

 

[DK]

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?

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

 

[Sheldon]

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.

Did this over two tanks after filling the tires. The bad mileage was tank
after tank after tank.

 

[Tony Hwang]

Did this over two tanks after filling the tires. The bad mileage was tank
after tank after tank.

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.

 

[BoB]

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.

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

 

[suburboturbo]

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- Hide quoted text -

- Show quoted text -

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.

 

[Todd H.]

My Subaru dealer was filling tires on all new cars with nitrogen and
adding $200 to the total cost.

$200 freakin dollars? Geeze. Ridiculous.

 

[Tony Hwang]

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.

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.

 

[Rick Courtright]

Does this sound plausible?

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

 

[KLS]

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.

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.

 

[Carl 1 Lucky Texan]

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

LOL'ing at 78% nitrogen mix!

hahahahaha!

Carl

 

[Frank]

LOL'ing at 78% nitrogen mix!

hahahahaha!

Carl

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.

 

[suburboturbo]

LOL'ing at 78% nitrogen mix!

hahahahaha!

Carl

Yup, been using that 78% nitrogen mix for years and agree that it
functions pretty well, both for street and race applications. If we
could only find some other use for that stuff. Of course, if all
depends on what makes up that other 22%. Now, if you really want to
"light 'em up", filling them with hydrogen would reduce rotational
mass, allowing for spectacular wheelspin and provide for some dynamite
burnouts.

 

[Carl 1 Lucky Texan]

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.

Hah! I wonder if they pull a vacuum before the nitrogen fill?

Carl

 

[John O]

$200 freakin dollars? Geeze. Ridiculous.

An entire cylinder of helium is only $100. Don't know about nitrogen,
probably cheaper.

-John O