What factors affect the maximum air pressure that should be put in a vehicle's tires?

Question

For just about every car, there's at least two different "maximum tire pressure" ratings.

• The vehicle manufacturer's rating. This is usually found on a sticker attached to the driver's door.
• The tire manufacturer's rating. This is usually found on the side-wall of each tire.

Given this, I've actually got two questions here. I'm fairly certain I know the answer to the first, but would rather the answerers here respond to confirm. The second is really the meat of things, though.

Which rating should be followed when filling one's tires? Should you use the vehicle's rating, the tire's rating, the lesser/greater of the two, or somewhere in between?

What variables can affect the maximum pressure rating that should be used for the tires? Could some major vehicle modifications, such as drivetrain or suspension changes, make the true "best" fill level deviate from one or both of the manufacturers' ratings?

EDIT: For specificity's sake, I added the safety tag. I'd like this issue more addressed from a safety/vehicle health standpoint, than a racing/performance perspective.

Answer 1

Bit of background: you can run your tires at various pressures, either under or over the recommendations. Over inflating the tires leads to overwear down the middle of the tire but a nice rigid tire - which is great for track days, smooth roads etc. Under inflating wears the outside edges more, and weakens the sidewalls meaning the tire can move about more - which is good on bumpy roads as they soak up bumps.

The tire manufacturer rating is usually a recommended 'do not exceed' from the manufacturer in order to avoid tire wear.

The vehicle rating is based on the weight of the car and the handling characteristics (including the suspension travel) so is more around handling/comfort.

As an example, my vehicle recommendation is 32psi on the rears and 34 at the front, but I use 36 at the rear and 38 at the front as I prefer the handling characteristics - it points more on corners and is slightly more prone to oversteer. I still keep it under the tire manufacturer recommendation, which on these ones is 44psi unless I'm on a track racing - then I crank em all up to 44/46. Those pressures on the road make it very skittish and uncomfortable.

(caveat - it is a performance car, and I err more on the side of precise control and less on comfort)

Update based on @Iszi's safety request:

If you are within the tire rating you keep the tire safe and reduce the risk of damage. As you raise the pressure, you make the tire harder and increase the pressure on the sidewalls which could be more likely to blow out in the event of hitting the tire off a kerb or pothole. Also, a higher pressure tire will place more load on the shocks/dampers/mounting brackets as it won't absorb so much of the impact from road bumps, so you can expect a shorter life for your shocks.

Realistically, unless you are pushing the performance, I wouldn't expect this to make a dramatic difference to safety or wear and tear. Keep pressures somewhere below the two maxima.

Answer 2

Follow OEM. The max rating that is on a tire is usually given to prevent people from over filling, and also it goes along with a weight limit. This means if it says 55, but 80 when maxed roll with 55. If pulling a heavy boat, then you roll with 80 because the extra weight would require the extra pressure for the tires to hold up properly. Bottom line, if you are simply driving your vehicle normally, stick to manufacturer recommended pressure.