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

[mountainwalker]

Thanks Nipper, in our case we're talking about just heading to snow country in the Sierra Nevada mountains once in a few weeks. Only the last 1/3 or 1/4 of such drives can have snow and we carry chains. We hardly get a flake of snow where we live, so we only use quality all weather tires.

[subaruplatt]

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?

 

I recently discovered that 35/32 at +3˚ c = 33/30 at -3˚ c with a 93 load index tire. I have found that the higher you exceed the stock load index, and mine was 89 stock vrs 93 replacement . The higher the load index the less likely you will ever need to over inflate past stock.

 

It is possible that your stock tire/load index was plenty capable with 30 psi.

 

If you want more psi then you will need different shocks.

 

I wouldn't be comfortable with nitrogen unless I carried it with me in the car, maybe it's better I just don't know. What's heavier a pound of nitrogen or a pound of air?

Edited January 9, 2012 by subaruplatt

[mountainwalker]

Is the load index noted on the tire or on manufacturers specs online? What does the load index translate to? 89 = ? 93 = ?