Varying front/rear pressure in a full time AWD car

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Apr 28, 2007
California, USA
I recently posted about low profile tires and was advised to bump up the tire pressure a bit when driving on rough roads, just to be on the safe side with respect to helping the tires keep the wheels safe. I know on these Subaru AWD systems, matching tire circumference between all the wheels is a big deal. Mine is a manual transmission model, with a full time 50/50 mechanical torque split. It's got a viscous limited slip diff in the middle and open diffs in the front and rear. I assume this means front/rear size split is important to keep the viscous coupling in the center diff happy. The door placard recommends 33 PSI front and 32 rear. My guess is that the difference is motivated by the fact that front tires will compress more than the rears under the load of this front-heavy car. Considering I'm only talking about going up a few PSI, I suppose it's probably safe to treat the 1 PSI difference as a constant and pump the tires to, say, 36 front 35 rear. In fact, that's probably the best I can do given the "accuracy" of tire pressure gauges. Does this sound safe or do I need to try to more carefully compensate for the difference? Seems like overkill but I guess I could get some string and measure actual wheel circumference by rolling the car.
I think it's safe if you increase pressure both front and rear by the same amount. It's more importance to maintain left and right tires of the same axle the same pressure.
It's going to take a LOT more than a few PSI difference to cook that center differential. I drove ~150 miles at highway speeds with a rear tire that had less than 15 PSI in it in my WRX. All of the other tires had 40 psi. Still kicking, and that was about 40,000 miles ago. To answer your question: Run them all at 40 psi, or stagger them 40/39, 41/40, whatever makes you feel good. Minute tire PSI difference isn't going to kill your transmission. Go to NASIOC if you want better information, but many people (myself included) have done exactly the opposite of the door placard and inflated the tires to something like 38 front/44 rear in order to quell a nasty-sounding transmission noise in our WRXs.
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According to the guys at the Forester Forum, Subaru supposedly has softened up some on their desire to have all four tires at the same rolling circumference. I ran my 06 Forester on the "1 lb. less in the rear" theory 36/35, 35/34, finally settled on 34/33. I found that the car's ride was noticably degraded by as little as 5 lbs. extra pressure. I also found that snow tires rode A LOT better than the A/S H rated tires.
I like running tires 2-4 over the door sticker's recommendations. Better tire life, handling, and real life fuel economy [even if it does not register].
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