# How much should I inflate my tire?

I have a Mitsubishi Lancer (but I don't live in USA, so it's not the same sold in America), and the manual says to inflate tires to 31 psi.

Now I'll be replacing my tires with ones from a different brand/model, which is rated by some costumers to be quieter, softer (less vibration from the road transferred to the car) and with a slightly smaller grip (but since I don't use it to the limits, I think it's ok).

Since they have some characteristics that are different from the original tire, how much should I inflate? The original recommended pressure (31 psi) applies only to the same tire, or it's a characteristic of the vehicle (and should be used no matter what tire I use) ?

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tl;dr: The recommended pressure is almost always a good general recommendation. You can make adjustments to suit your specific needs.

The original recommended pressure (31 psi) applies only to the same tire, or it's a characteristic of the vehicle (and should be used no matter what tire I use) ?

As a first step, I'm going to point you at Tire Rack's excellent discussions on tire pressures and a whole host of other technical topics.

31 psi is a reasonable starting pressure on your vehicle, regardless of the specific tire that you put on it.

Let's quickly consider some of the factors that the air in the tires affects:

1. It acts as a spring. We don't drive on solid rubber tires for a reason: these are much more comfortable.
2. It defines the tire's shape. More air => taller tire with a smaller contact patch. Less air => shorter tire with a larger contact patch.
3. It defines the tire's stiffness, especially in terms of the sidewalls. An underinflated tire is going to roll over its sidewall in a hard turn, wearing out the side and edge. A well-inflated tire will hold its shape, keeping a more defined contact patch on the road.

Now let's talk about examples of where we'd potentially like to adjust our tire pressures. Note: all of the factors below assume that you're tuning by relatively modest amounts (e.g., plus or minus 10%). So when I say "more air," I don't mean 100psi, and "less air" is not the same as saying "zero air."

1. A front engine car is heavier in the front than it is in the rear. It is common to see a higher tire pressure in the front partially as a consequence of this increased load.
2. Street cars are generally set up with a bias towards understeer. Lower tire pressures in the rear will increase grip on the back end, reducing the chance that it will swing around on a casual driver.
3. A tire is an undamped spring (i.e., no shock absorber). A high tire pressure will transmit more of the bumps and jolts of a lower quality road directly to the driver. This factor increases in importance in proportion to the annoyance of the spouse in the passenger seat.
4. A tire with less air will generally have more grip in all respects. This means that it will feel less responsive to sudden steering adjustments. A small drop (e.g., 2 psi) is common to increase winter traction where real snow accumulation is common and persistant (i.e., not where I live).
A severe drop in pressure will cause the sidewalls to bulge out significantly and dramatically increasing the footprint of the tire. This can be used to get out of a deep hole in the snow. You can see a practical example of this technique in the Top Gear Polar Special.
5. A tire with more air will have a smaller contact patch and stiffer sidewalls, generally leading to increased fuel economy.

In short, tuning the tire pressures is the absolute cheapest way to adjust your suspension performance.

Nitpick: yes, I know that it's not the air alone that's acting as a spring. It's the combination of the air, rubber, belts and the deformation that happens under an impact. For an introductory discussion, though, we can say "air = spring."

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Addendum to #4 : Dropping tire pressure down a bit (usually 2psi or so) below the car manufacturer's number to improve traction is fairly typical in countries that get actual winters (e.g. Canada). Heavily deflating the tires (to the point where the sidewalls bulge) is also a handy trick for getting unstuck. – Compro01 Dec 13 '13 at 14:24
@Compro01, Good point - I'll add that above. – Bob Cross Dec 13 '13 at 16:21

The tyre pressure given out by the vehicle manufacturer is determined after many thousands of miles of testing, together with the tyre manufacturer, to arrive at optimal tyre performance. It should be used and adhered to.

The correct tyre pressure ensures the full contact area of the tread is maintained with the road. Under pressure will allow the tyre tread to lose 'full' contact with the road. The flexing of the tyre walls bends the centre tread area away from the road. Proof of this is tyres that have both of their edges with tread worn away whilst the centre part of the tyre remains a lot less worn on under inflated tyres. Under inflated tyres will also allow the outer tread area to 'shuffle' excessively and this will reduce road holding.

Over inflated tyres will wear greater on its central tread area and this can be seen by a worn out centre section with the outer edges of the tread very much less worn. The over inflation will also stiffen the side walls and lessen the required shuffle of the tread causing the tread to scrub, as well as giving a harsher ride.

The correct tyre pressures will maintain the correct operation of the tyres giving the best life, performance, and safety for the vehicle. Also the correct pressures will maintain the correct rolling resistence giving best fuel economy, this also will reduce vehicle emissions due to the lesser fuel consumption.

The decal on your door jamb or petrol filler cap will give you the correct tyre pressure for your vehicle. Use that pressure. The difference between one premium quality tyre from one manufacturer and another will be minimal and is not a real consideration. An obscure or unknown brand selling cheaply should be avoided, even if they have a type approval.

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