Should a different tire size mean different air pressure?
 

I've been wondering this for awhile, and tried searching this forum and google, which led to more confusion.

I've seen some authoritative sources say that sizing up a tire means you need less pressure, while others say you need more.

The most convincing answer I've seen says there should be load to pressure charts from the tire manufacturer, but I can't seem to find any.

For reference, I'm running 245/40R17 Pilot Super Sports. My intuition and observations seem to indicate I should be running them at lower pressures than the stock 34PSI, as I know that the donut tires are routinely 60+PSI and the last set wore down the middle much faster.

Yes, to keep the same contact patch area you should adjust your pressure:

New Pressure = stock pressure * (Stock tire width / New tire width)

This assumes the tires are approximately the same overall diameter.

-Josh

The load rating is the number after the size of the wheel. Example 245/40R17 97W. 97 is the load rating, W is the speed rating.

You need a certain air mass to support your car. You can get that mass either in a small tire under high pressure, or you can get it in a larger tire (and subsequent larger volume) with lower pressure, but still holding the same total mass. All other things being equal, pressure and volume are inversely proportional. Yay, science.


Your larger (wider) tire now has more volume than the stock tires. There are other variables to consider, such as differences in construction and compound, but generally speaking, you'll likely need less pressure for the tire to work correctly. Now, if you have a 245 squeezed onto a 7 inch rim, then you'll need more air pressure to help support the tire from rolling over when cornering than you would if the tire were mounted on a more optimally sized rim, but for just pure load-carrying capacity, the tires will need less pressure. Get some chalk and experiment.