Your tire pressure on 19" wheels

[XMetal]

Guys, what tire pressure(s) are you running on your 19s? Just curious...

[The HACK]

IMO, tire pressure doesn't change no matter what size rim/tire you go to. Tire pressure dictates the size of the contact patch, and while the width of the tire will technically change the shape of the patch due to the stiffness of the sidewalk, the diameter of the wheels's effect on the contact patch is negligible.

So if manufacturer's recommended tire pressure is 32psi (for sh*s and giggles) on a 255mm wide tire, and the weight placed on that corner of the car is, oh, 850lbs, that pressure would yield a 26.56 square inch area for contact patch, or roughly an 8" x 3.2" patch. That's why they tell you that basically what keeps you on the road is about 4 patches the size of your hand, or a piece of letter sized paper (legal sized for larger, heavier cars on under inflated tires of course).

That's an over-simplification of tire dynamics and physics, but the basic principle does not change when you go to a larger wheel size.

[XMetal]

I just picked up an S3 with 19s and was shocked to see the recommended tire pressure of 44psi I thought the freaking dealer forgot to air it down after delivery when I first measured it, until I looked at the door jam. The S3 only weighs about 300lbs more than the Mcoupe, but wants a 14psi more for those extra 300lbs? This is the highest tire pressure I had ever seen recommended on a passenger car. For reference, my 5000lbs X5 (also have 19s) at maximum carrying capacity (GVW) of 6000lbs only needed 39psi on the rear. Now that got me really curious as why Audi wanted to run such a high tire pressure on the car.

[The HACK]

Originally Posted by XMetal

Now that got me really curious as why Audi wanted to run such a high tire pressure on the car.

CAFE.

[XMetal]

That makes sense for CAFE, but I'm certain that high of pressure will cause wear issues when going away from the OE spec tires. May be that's why the OE tires looked weird and stretched - it's got 235/30-19 on an 8" wheel, but looked as if a 185 tire is mounted on it.

[The HACK]

Bean counters have already determined that the cost of paying CAFE fine is less than accelerated wear on tires, because most manufacturers warranty does not cover tires.

So your tires wear out the middle in 18 months, cost VW Group nothing. Increase fleet fuel economy by just 1 mpg saves them HUNDREDS of millions. So what if overinflated tires create a smaller contact patch? Every crashed car in an accident is a new sale for SOMEBODY.