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Optimal tire inflation for 2003 Outback VDC 3L H6? Subaru recommended way too low


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#1 mountainwalker

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Posted 15 October 2011 - 01:20 PM

Hope you can help with advice on an optimal inflation level for our driving conditions given that the recommended Subaru numbers seem way low and deliver lower mpg.


The recommended tire pressure for our 2003 Outback VDC 3L H6 is 30 PSI front, 29 PSI rear on the sticker in the driver's side door well. That seemed way low and delivered lower mpg and poorer/softer handling. However, a nearby Subaru dealer recommends adding 1 PSI for each additional 40 lbs., whether passengers or cargo.

My wife is usually with me for 50% of driving during weekdays and nearly 90% on weekends, and that adds another 130 lbs, not to mention we’re either hauling groceries on weekends or taking two friends on hiking/backpacking/cycling trips (add another 300-350 lbs in the rear seat) and either a bicycle hitch (add 35 lbs) and two bikes (add 50 lbs) OR hiking/backpacking gear (add 40-100 lbs depending on trip and number of people).

In addition, 95% of our driving is in the mild close to sea level San Francisco Bay Area, dry for 6 months out of the year with sunny daytime temps of 75-85F and nighttime of 55-60F, and winter daytime temps of 50-65F with alternating sun and cloudy rainy windy days and winter nighttime temps of 35-45F. We take winter trips into higher snowy elevations in Tahoe, Yosemite and other locations, up to altitudes of about 8000 feet.

Our high performance tires can be inflated up to 44 PSI. We typically inflate with Costco blend of 90% nitrogen and 10% oxygen so that the tires are less prone to expansion and contraction due to changes in weather.

The Subaru recommended number seems way low, and the car gets lower mpg at this inflation and softer/poorer handling. On the advice of one local Subaru dealer service guy, we inflated all the tires to 34 PSI. Found this even a bit low, and inflated to 36 PSI in all tires, which of course gives us better mpg.

Questions:

1) What would be the optimal tire inflation level for us for late spring, summer and early fall in our area given our weekday and weekend use and not having to constantly adjust tire pressure up for weekends and down for weekdays?

2) What would be the optimal tire inflation level for us for late fall, winter and early spring?

3) Should we be inflating 1 PSI lower in the rear following the original Subaru recommendation in on the sticker in the driver’s side door well?

4) Based on the recommended typical inflation for our regular conditions, should we inflate the tires more for venturing into the snowy mountains, and if so, how much more?

5) Will the Costco 90% nitrogen and 10% oxygen mix contract much in cold weather in the mountains?

6) Why would Subaru have originally recommended such a ridiculously low inflation of 30 front and 29 rear?

#2 Suzam

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Posted 15 October 2011 - 01:35 PM

I have friends in Colorado just outside of Denver who routinely drive between 5,000 and 10,000 ft altitude.

They use Michelin tires from Costco inflated with nitrogen on a Tribeca and run 32/30 front to back in the winter and 36/34 in the summer. The tread does wear quicker in the center because of "over" inflation but not really noticeable until the end of the tread life. They have close to 200k on the car now and have done this since new.

They do raise the rear pressure 2lb when they carry more people or cargo for any distance.

In Colorado, it seems like everything is a good distance from everything else. It seems like every time you get in the car you have to travel 50 one way miles just to get to were you want to go. At least that's how I feel when we visit.
:lol:

#3 rverdoold

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Posted 15 October 2011 - 01:58 PM

At sea level here and temperatures between 40 and 85 F I try to have between 37 and 39 PSI in 205/55 R15.
At winter between -30 and 40 F I have between 34 and 37 PSI, if I need more grip I reduce to 28 to 30 PSI in 185/65 R14.

I check sub-weekly since I have notice one of my summer tires is deflating slowly. And it makes a big bang on the fuel economy!!
My summer tires have a max inflation of 55 PSI stated on the side. I assume that above that = over inflation. I am doing this for the last 5 years and never seen weird tread wear on any tire.
This on a 1999 impreza bi-fuel so extra 60 to 80 kg on the rear axle for the propane tank.

Edited by rverdoold, 15 October 2011 - 02:03 PM.


#4 Fairtax4me

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Posted 16 October 2011 - 12:54 PM

I typically run 35psi Front and 33 - 35psi Rear in both of my Subarus. Load dependent. I don't remember what the recommended pressure is but it's somewhere around 3 - 5 psi lower.
30-29 does seem really low for a wagon with the 6 cylinder engine, but inflation pressure is also dependent on tire load rating. A tire rated for a higher load will need slightly less pressure.
But if you're comfortable running the higher pressure, and don't have significantly uneven treadwear, go with it.

#5 grossgary

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Posted 16 October 2011 - 02:06 PM

run 35-40 psi. it's just tire pressure, it's not that big of a deal. increase it and you're golden. most people that know anything about cars do not run the factory/dealer spec's, they're always to low.

#6 Suzam

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Posted 16 October 2011 - 05:46 PM

Whatever you choose, monitor the tread wear from center to outer edges of the tire. If you're wearing out the center faster, then your contact patch is smaller than the tread width.

Ideal pressure for the tire would be even wear across the tread for the life of the tire. Over inflate too much and you loose more of the contact patch which is surprisingly small to begin with.

#7 unibrook

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Posted 19 October 2011 - 03:03 PM

2001 Forester 5MT. 110k. I have always run 35lbs all the way around.

#8 heartless

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Posted 20 October 2011 - 09:18 AM

i typically run 32 psi all around on my 90 Legacy wagon (14" wheel size, just slightly above sea level - roughly 200ft +/-)

Ideal inflation will vary greatly depending on location, weather, load, etc. and is somewhat subjective - what works for one, may not work for another.

to me, ideal inflation gets me good fuel mileage without compromising contact patch - which, as already noted, is rather small to start with.

contact should be even all the way across the width of the tire tread
overinflation causes more wear on the center part of the tread,
underinflation will cause more wear on the outer part of the tread
the trick is to find that happy medium where there is even wear across the entire width of the tread.
for me, that happens to be right around the 32-33 psi mark - i can tell if i have a low tire (very slow leak) by fuel consumption. :o

If i know i will be hauling more weight than "normal" (normal being the usual grocery trips, and the occasional home improvement project supply run), i will bump up my pressure by a couple of psi for the duration of that haul (such as helping someone move)

#9 CNY_Dave

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Posted 21 October 2011 - 12:43 PM

I'm often at 44/43 to get the treadwear even across the face.

Dave

#10 Mike104

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Posted 21 October 2011 - 01:29 PM

We all remember what happened when Ford Explorers ran with lower tire pressure that was recommended by Ford and lower than Firestone wanted:

http://en.wikipedia....ire_controversy

That issue was one reason why cars now have Tire Pressure Monitoring Systems.

http://www.nhtsa.gov...e-fmvss-138.pdf

Two main considerations are to not inflate the tire pressure COLD above the maximum rating on the tire sidewall and to inflate the tires for equal wear across the surface as mentioned above.

#11 mountainwalker

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Posted 05 January 2012 - 03:48 PM

Thanks everyone, all of this was very helpful. I experimented and I found that 37 and above (hot pressure) and you could be dealing with more squirrely handling on turns. 36 hot and below was fine. If you go with the Subaru recommendation (with no extra passengers/weight) you will lose a lot of gas mileage - at least 2-3 mpg.

I called one of our local dealers and spoke with a service guy who seemed to have a clue, and he recommended a blanket 36F 34R hot (34F and 32R cold). I would say this recommendation of 2 PSI difference between front and rear, or just 1 PSI difference between front and rear (36/35 hot or 34/33 cold) is about right, at least for our local weather in the San Francisco Bay Area.

I also found I can't rely on any gauge except my own high accuracy one. I had Costco fill to 36/35 hot - checked later after car idle for the night, at the same outside temperature, and all tires front and back read 34 cold. So I drained the rear tires to 32. I think keeping all the tires at the same pressure would definitely put stress on the drive train.

My only remaining question - with heavier loads such as below (two common scenarios for us), how much if at all would you bump up pressure from 36F/34R hot (34F/32R cold), given that Subaru recommends increasing PSI by one for every 40 lbs extra load:

-Backpacking or car camping or ski trip with 3 passengers at average 175 lbs per passenger) plus 35 lbs each in gear; Total 630lbs

-Bicycle trip with one passenger (135lbs), bike carrier 40lbs, two bikes at 30lbs each, and 20 lbs in food and gear; Total = 255 lbs

#12 mountainwalker

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Posted 05 January 2012 - 07:40 PM

Howard, a respected service manager at the best Subaru dealer in our area (which LOL I found about today and not before), recommends cold 34F/34R to handle every day driving with just the driver, as well as a second passenger and some groceries from time to time.

For loads of 3 adult passengers and gear up to 600 lbs, or one adult passenger and gear up to 250 lbs, he recommended just going up 1 PSI for the trip. Seems a bit low to me for the 600 lb load, but Howard worries about over inflation, especially people forgetting to let the extra air out after the trip.

#13 bheinen74

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Posted 05 January 2012 - 07:49 PM

34/32 rear


or 35/33
something about that, anymore the ride suffer lots on bumps.

#14 mountainwalker

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Posted 05 January 2012 - 09:46 PM

34/32 rear


or 35/33
something about that, anymore the ride suffer lots on bumps.


bheinen74, if you mean 34/32 or 35/33 cold, do you think maintaining the 2 PSI difference between front and rear is important?

According to the good dealer service manager (he was really sharp and responsible), you are going to carry an extra passenger and/or some cargo in the back enough that you'll want 34/34 cold in there for every day use - no less than 34/33 at least.

He also said the PSI difference between the front wheels under the heavy engine and the rear wheels didn't matter because the drive train wouldn't be hurt, because it has a center differential (his words not mine). Make sense?

#15 MilesFox

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Posted 05 January 2012 - 11:30 PM

I like to go by a 10% rule. For example, i the tire is rated ad 35psi max, then 10% is 3.5. now subtract that from 35 and you get 31.5 essentially 32 psi.

A tire rated at 44 psi take 4.4 away and you get 39.6 essentially 40 lbs.

I like to run 40 psi in my legacy wagon with 44 max. tires on it.

You can also determine the right inflation for the tire with a good eye, if you can look along the contact patch from the front or rear of the tire and see how flat the tread lays. If it us under, the middle tread will be cupped inward, and if over inflated, the sides of the tire will come off the ground.

If you are recording your tire pressures, and varying them for trial purposes, you can observe the wear patterns on the tires between logs to determine if over or under is the way to go. If thetire is wearing in the middle, it is over inflated, if on the outer edges, under inflated.

#16 1 Lucky Texan

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Posted 05 January 2012 - 11:52 PM

for 'everyday use' I add 10% but keep the 1 psi difference. So about 33/32 or so.

for extended trips with a load - I might add a couple more psi. As stated above, too much and the contact area is diminished. And you need good braking precisely because you are heavily loaded!

#17 dsmcolt969

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Posted 06 January 2012 - 12:59 AM

35psi should be perfect for all conditions, you wont really have any additional weight in the car to cause any pros/or cons with extra weight for shirtish trips. and just so you know i hope costco isnt charging you for this special "nitrogen" air because a 90/10 mixture is just the air around you that you breathe everyday. but what do i know i just work with tires everyday

#18 mountainwalker

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Posted 06 January 2012 - 01:10 AM

35psi should be perfect for all conditions, you wont really have any additional weight in the car to cause any pros/or cons with extra weight for shirtish trips. and just so you know i hope costco isnt charging you for this special "nitrogen" air because a 90/10 mixture is just the air around you that you breathe everyday. but what do i know i just work with tires everyday


LOL no they don't charge for air, and we only go there when we have to shop for regular things.

By 35 PSI do you mean 35 cold or 35 hot? If 35 cold (which is roughly 37 hot), we found the ride gets hard/jarring above 34 cold (36 hot). Also you can lose a bit of traction.

I'm finding 34 cold (36 hot) is about optimal balance of fuel efficiency/ride/traction when not carrying heavier loads.

What did you mean by "you wont really have any additional weight in the car to cause any pros/or cons with extra weight for shirtish trips." For a camping or backpacking trip with 3 adult passengers in addition to me, that's over 600 lbs extra that I don't lug around every day. And a biking trip with one passenger can be an extra 250 lbs I don't lug around every day.

Outside of trips my typical load when it's not just me is one adult passenger 135-180 lbs and groceries/supplies of about 80-100 lbs.

#19 dsmcolt969

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Posted 07 January 2012 - 01:26 AM

im saying the amount of driving in your daily activities and the little bit of driving on your trips isnt worth the hassle of changing the tire pressure for. but if you can truly tell the difference between 1-2 psi in your tires then obviously it is worth the hassle for you

#20 screwbaru2

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Posted 07 January 2012 - 03:22 AM

My wife's OBS H6 2003 is 35/35 year round all conditions. Why? Because on the tires wall is embossed "inflate to 35 psi". Drove 2000 miles on vacation last June 1600 miles with 4 scuba tanks dive gear, luggage and my wife. No problems, with the car anyway.

#21 bheinen74

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Posted 07 January 2012 - 02:01 PM

There are a few engineering reasons why Subaru specifically recommends slightly more pressure in the front tires than back:

does anybody know why this is important?

It is because there is more weight in the front of the car typically, the engine and trans is up there so yeah, more weight.

And we all know Subaru ires have to match in diameter so the transfer clutches do not see excessive wear.

So a lightly higher weight in front, you need slightly higher pressure to keep the rolling diameter the same.

If you do no follow the prescribed engineered higher pressure, you are essentially doing the same as if you had mismatched tires.

they have engineered those PSI levels and posted it in the manual, and on the door sticker. For a reason. Engineers re not stupid (well i can say in general they are not)

Same reason they say if you have high load weight, to increase the pressure to specified.
You really do not want equal pressure on all 4 on a Subaru. You are damaging the drivetrain if you do.

We should have a sticky on "mismatched tires and tire pressure"

#22 mountainwalker

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Posted 07 January 2012 - 02:53 PM

There are a few engineering reasons why Subaru specifically recommends slightly more pressure in the front tires than back:

does anybody know why this is important?

It is because there is more weight in the front of the car typically, the engine and trans is up there so yeah, more weight.

And we all know Subaru ires have to match in diameter so the transfer clutches do not see excessive wear.

So a lightly higher weight in front, you need slightly higher pressure to keep the rolling diameter the same.

If you do no follow the prescribed engineered higher pressure, you are essentially doing the same as if you had mismatched tires.

they have engineered those PSI levels and posted it in the manual, and on the door sticker. For a reason. Engineers re not stupid (well i can say in general they are not)

Same reason they say if you have high load weight, to increase the pressure to specified.
You really do not want equal pressure on all 4 on a Subaru. You are damaging the drivetrain if you do.

We should have a sticky on "mismatched tires and tire pressure"


This sounds perfectly logical. However, the Subaru recommended inflation numbers in the door of the vehicle - 30F and 29 rear cold, are way too low for a good balance of fuel economy, tire wear, handling under different weights (one driver no cargo versus one driver 3 passengers plus cargo), etc. Also the Subaru door sticker says the rear tires are at 32 PSI when in towing mode (I'm assuming this means you should be at 32 PSI cold rear when towing, which I think is too low, especially with passengers in the back). Without passengers in the back 30F and 29R are low, but with passengers and cargo, you are going to ride too low in the back and handling is going to suffer.

Subaru also recommends adding to the above number 1 PSI for every 40 lbs increase in weight. That bumps you up ridiculously high if you follow it to the letter.

So whether its the engineers or the business folk or the lawyers at Subaru, someone is fudging the math.

Thus the best rated Subaru dealer in our area fills all Subaru Outback tires, front and rear, to 34 PSI cold. They do this as a best practice to account for the fact that most drivers will drive under a variety of weights - again single driver, versus driver plus occupants plus cargo. The service manager there said he's recommend 34F 34R cold, or 34F and 33R cold. He said having all 4 wheels at the same PSI would not harm the drive train because of the central differential. I'm just repeating his words, not saying he's right. He added that for trip driving under heavier load (say 3 adult passengers plus 150lbs or so in cargo) he would add 1 PSI to the 34F and 34R cold recommendation, but reduce that after the trip.

Keep in mind that our area sees fairly moderate winter temperatures. Winter lowland 32F minimum at night and 65 max daytime, and winter mountain ski area 15-20F at night and 40F typical daytime.

Based on my experimentation and the above conflicting Subaru door sticker and Subaru dealer guidelines, I'm thinking that 34F and 33R is best for most daily driving, when driving by yourself or with just one passenger upfront, to account for engine weight.

And that for a weekend trip with one passenger and light cargo you could stay the same, or with a bit more cargo or a rear hitch bike carrier bump to 34F 34R, and for a fully loaded trip with 3 passengers and cargo, you're best at 34 or 35 all around.

#23 mountainwalker

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Posted 07 January 2012 - 02:56 PM

How much would you bump down tire pressure for driving on snow?

For driving on snow at 8-9,000 ft altitude?

And for driving on snow fully loaded with driver, 3 adult passengers and cargo (say 175 lbs driver plus 650 lbs passengers and cargo)?

Love to hear from people who live or often drive in mountain and snow country.

With this I'm more than squared away on Subie Outback tire pressure LOL.

#24 nipper

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Posted 07 January 2012 - 06:22 PM

run 35-40 psi. it's just tire pressure, it's not that big of a deal. increase it and you're golden. most people that know anything about cars do not run the factory/dealer spec's, they're always to low.


It is also the easiest way to blow a tire, wear out tires, or make your handleing really weird. It is not "just" tire pressure.

#25 nipper

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Posted 07 January 2012 - 06:31 PM

If you drive someplace with a lot of snow or a fully snowy winter (say anyone seen any of this "snow" this year?) you should have dedicated snow tires.




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