Different front & rear tire pressures

The STi recommends 36 in the front, and 31 in the rear. The Subaru tech who works on it (and also races) told me that it is to balance the car properly and reduce understeer. The camber angles are different on the front and rear wheels to improve handling. In addition, most of the weight (the engine) is in the front, so the increase in front tire pressure is intended to reduce the spring rate and balance the car overall.

They told me that matched front and rear pressures is not bad on Subarus in general, but the car will have a tendency to understeer. Many dealers want that since understeer is much easier to drive out of than oversteer. If there is too much pressure in the front tires, the car will oversteer though.

Tires are supposed to sit on the road a certain way...too much air and the center of the tread is weighted more so the center wears faster and there is also less traction. Too little air and the edges of the tread wear plus sidewalls squirm and the tires run hot. Since the weight on the front and rear axles is different the air pressure has to be different as well. My pickup exaggerates this. The engine, trans, and cab are all on the front and an empty box is on the rear...I need to run 35 in the front and only 28 in the rear to get good tire wear and handling. TG

These are the same numbers as the tire sticker for my '04 WRX (P205/55-R16 tires). Many front-engine cars come with similar recommendations for more pressure in the front tires, as they have more weight on them. I'd imagine the recommendations for a car with a near 50/50 weight split (think mid-engine) might be different.

Hydroplaning risk is a combination of tread depth, tread design, tire pressure, tire width, and weight on the axle. I've even heard that the risk of hydroplaning (for front-engine cars) is reduced if tires with deep grooves are kept on the rear. However, that conflicts with the usual desire to rotate tires such that they will be replaced as a set of 4.

Strange things happen.

My rule of thumb for years has been rwd, start with 2psi higher in the back, fwd 4psi higher in the front,. awd, equal pressures all round; then set pressures to match your driving style and how you want the car to handle from there. Obviously don't exceed the tyre manuf recommended pressures though. If you find a setting you like, accept any higher tyre wear that results as a penalty for enjoying yourself! Years back we ran an Austin 1800 with 42f, 26r and almost completely removed any understeer - mind you, the resulting liftoff oversteer was rather entertaining, especially on a wet road .... For what it's worth, my 91 Liberty (Legacy) wgn ran 32f, 35r under most conditions for 200000+kms and my 99 OB ltd runs 36 all round & is reasonably neutral. Cheers