I also see what Johnny is saying.... and it initially makes sense from a pure physics perspective.


However, Subaru of America disputes his claim.

They recommend a 4 psi pressure delta between the front and rear tires for this reason...

Since the front of the car is heavier, more pressure will be required there to maintain the same rolling circumference as the rear tires....

Subaru indicates this very clearly and quotes that center diff damage will result if this rule is not followed.


People with WRXs have actually eliminated annoying drivetrain shudder on deceleration by resetting their tire pressures accordingly.

I can bring up the threads on the subject from NASIOC if you are interested.



Think about it this way... sure, the tire itself is not changing circumference... however, the part of the tire that is touching the ground IS closer to the wheel hub and therefore the ROLLING diameter of the wheel/tire combination has changed. All parts of the tire that are not touching the ground at any given moment become irrelevant to the physical problem. As the tire is compressed at the point where it touches the ground, it is in effect the same as putting a smaller tire on with full pressure... the distance from the hub while rotating is the same. Therefore, the overall axle ratio to the ground has changed in the back vs. what is happening in the front, causing a differential in rotation where the shafts intersect in the center diff. The diff is forced to constantly slip ever so slightly in order to allow this to occur. This causes premature wear on the plates and a huge increase in heat produced in the housing. This may be one of the things that contributes to high transmission temperatures in the SVX... incorrect tire pressure delta.