Brickboarders have recommended variously: (thanks to all for sharing experience)

Sticking with the original pressure plus a couple of psi for normal use, to plus six to eight psi when loaded (the brick!).

Running 35psi on all 4 tires and adding a few psi when fully loaded, on the basis that most tire failures are due to low pressure, which leads to high heat that breaks down the tire itself.

Experimentation above the Volvo recommended figures up to the maximum inflation pressure of the tire in use - based on handling, wear etc. and then increasing for load or high speed travel.

For those wanting to determine the figures from available data, the method and theory posted by KEN C goes like this:

If you're primarily interested in maintaing the proper load rating for your car's weight, here's what to do.

First it has to be said that in virtually every decent tire store, they will have behind the counter a reference manual (published annually) called "The Tire Guide" -- it's meant for the salesmen and techs, not the customer, but ask for it anyway. It's an industry publication (retail tire sales) that contains, among other important facts, a series of tables for every conceivable variation in tire size, that gives the Load Rating (lbs) of a given tire over a range of pressures (psi). Remember that the load rating of a tire that is stamped on the sidewall is ONLY the load rating at the Maximum Inflation pressure, and is not the load rating at other lower pressures, such as those specified by a car manufacturer such as Volvo. For example, you may have tires with a 42 psi maximum inflation pressure, but if the car manufacturer specifies 30 psi and you keep the tires at that pressure, then the tire is not really suitable to carry the "stamped on sidewall" load, but some lesser load that the manufacturer deems adequate (except in the case of Ford Explorers :-).

Now, here is what you do. You take the Tire Guide book, and look up the table for your original tire size, and note the load rating at the pressure you are using with your old tires (if you use different front and rear pressures, note the two load ratings).
Next, go to another table for your new, different tires' size. Now scan the table for the load rating you noted from the previous table, and see what tire pressure corresponds to that same load rating. This pressure is the one to use with your new tires in order to ensure that you have the same load rating.

Of course, you can use a higher pressure, to give you a higher load rating, but with this method you at least are ensuring that your load rating hasn't changed (diminished) with your new tires' size.


Owners of older cars, faced with this question, may have a later car which came from the factory with tires that have a new type of European "load rating" stamped on them – this means the nomenclature such as "87H" or "91V", i.e., a combination of two digit number and letter – in which case the situation is far easier and there's no need for a Tire Guide book.

First, refer to the two digit number, such as "87" from "87H" [the letter is a speed rating, not a load rating, btw]. Any tire with the same two digit number such as 87 has the identical load rating at identical pressures, regardless of tire size, be it section width or aspect ratio or diameter. In other words, the two digit load rating factors in all everything affecting load carrying capacity and generates a single measure of the load rating. So for any two tires marked the same (e.g.) "91_", even if one is a .45 aspect ratio and the other is a .75, or one is a 13" tire and the other is a 17" tire, or one is 165 mm wide and another is 245 mm wide, these two tires have the identical load rating at any pressure.

Now, commonly, one tire may have a maximum pressure of 52 psi, and another may only have a 36 psi, but they will have the same load ratings anyway as long as they have the same two-digit number. What happens with lower aspect ratio tires that have higher maximum psi is that their load ratings do NOT continue to increase with higher pressures but rather reach a limit of their load rating (at a sub-maximum pressure, usually around 36 psi) even though their allowable pressures (up to their maximum stamped on the sidewall) allows their pressure to go higher. In other words, a tire with a higher maximum pressure isn't rated to carry more load than another tire (with the same load rating number) with a lower maximum pressure.
What the higher maximum pressure does provide is a capacity to tolerate higher speeds. You will find, for such tires, that they are speed rated so that a speed, e.g., over 100 mph calls for +4 psi, over 120 mph calls for +8 psi, etc. Having the higher maximum pressure rating allows such adjustment without exceeding the maximum inflation pressure.

Not everyone will be comfortable with this information – and Ken C said it went against his intuition, until he read this information written by a tire engineeer. It is true it seems.