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Old 07-07-2004, 02:08 PM   #1
Fermi
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Default proper inflation when car is really loaded down

After reading all the strict warnings from this website and Subaru about tire pressure affecting the differentials, I'm trying to figure out how I should have the front/rear tires inflated for an upcoming road trip in the mountains. A couple questions:

1) The car will be pretty loaded down in the back, so I assume this might change the -3 psi F/R differential since that is for an unloaded car? Should I actually measure the tires to get the diameters the same?

2) I'll be driving from ~600 ft above sea level to 8000+, so I assume the tire pressure will be higher when I arrive due to the lower pressure outside. Should I then deflate the tires to the original desired pressures?
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Old 07-07-2004, 02:20 PM   #2
bluesubie
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I believe the info you've read about tires affecting the diffs is mainly related to replacing one worn tire. IMHO, a load isn't going to change the circumference in the tires enough to worry about diff damage.

You want extra air in your tires to help support the extra load that you'll be carrying (to avoid blowouts, etc.). I've gone with 4 psi's over recommended psi's for trips with two weeks worth of luggage and toys including a Thule cargo box.

It's worth keeping it at 4 psi's over at all times anyway for better handling.

-Dennis
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Old 07-08-2004, 11:25 AM   #3
imprezzakc
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To be safe I would suggest to not exceed the max pressure on the tire sidewall . That is the max cold PSI so the tire can carry its max load . If you feel the tire is not up to the load you may want to consider a tire with a higher load index rating .

Just my .02
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Old 07-08-2004, 12:11 PM   #4
fliz
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Quote:
Originally posted by bluesubie

It's worth keeping it at 4 psi's over at all times anyway for better handling.

-Dennis

Not to mention better gas mileage.

Run a tank at 32/29, then run a tank at 40/37. You'll get better mileage at 40.

PS. I ran 35/32 for a while, then I put on tires that were inflated to 40/37 and got much better mileage. Now I run 42/39.

Re #2: The difference in atmospheric pressure between sea level and 8000 feet isn't enough to make a noticable difference in tire pressures. Atmospheric pressure is 14.7psi at sea level, 10.4 psi at 2500m ( http://www.personal.usyd.edu.au/~gerhard/pressure.html ) . You would cause more problems by dropping the pressures, because you would end up with underinflated tires when you dropped back down.

Last edited by fliz; 07-08-2004 at 12:18 PM.
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Old 07-08-2004, 04:20 PM   #5
Fermi
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Quote:
Originally posted by fliz
You would cause more problems by dropping the pressures, because you would end up with underinflated tires when you dropped back down.
Good point!
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Old 07-09-2004, 02:40 AM   #6
Uncle Scotty
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A LARGE percentage of problems that people have with tires are due to underinflation......and very few are due to over inflation.
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Old 07-09-2004, 02:58 AM   #7
ripvw
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as far as load capacity, P-metric radials reach their max rating at 35psi, Euro-metric radials at 36psi cold pressure. Putting in more air beyond that won't increase the load capacity of the tires. However, increasing pressure to at least that point guarantees that your tires are at their maximum load capacity

I like cold pressures between 35 and 40psi for their impact on dry handling. over 40psi you may be losing max contact patch width - i.e. dry cornering grip. you end up sacrificing grip for better "feel" and responsiveness - not a good trade in my book.
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