Troy H

"Some people complain about fast tire wear. Pay attention to the max pressure. If your OEM tire max pressure was 44 psi max and you replace it with 51 psi max (with the same load rating), you need to scale the operating pressure as well. Say your car OEM sticker says 33 psi. The new pressure for the 51 psi max rated tire is 33*51/44 = 38.25 psi. If your new tire load rating is different then the old one you need to factor it in. Say, if your old tire was 94H (1477 lbs max) and the new one is 95H (1521 lbs max) , the calculation is 33*51/44*1477/1521 = 37.14 psi. You will get better milage and longer tread life."

This is from a review of a tire I'm shopping. Now, the part about the load rating I can completely understand, but why should I adjust from OEM to aftermarket based on the max pressure stamped on the sidewall? The tire needs the same contact patch and pressure to support the weight of the car regardless of whether the max pressure is 44 or 51 psi, right?

TurboBob

but you're not taking into account that each tire has a different construction, which is the most important factor in correct tire pressure. Contact patch is only 1 part of the equation. For example, changing load ratings. A higher load rating tire is, by design able to carry more weight, it willl generally be a stiffer tire with more or thicker plys, and require higher pressure to retain correct shape when in motion. If the pressure is run too low, say to save the contact patch, the tire may distort more than it should, which can cause failure. This is a common problem for guys like me in construction, who need a very high load rating tire, to be safe when loaded down, but run light most of the time. I and others have killed tires by running a lower pressure when light in order to get correct wear, only to find blisters or even belts sticking out of tires in a short time. The tire manufacturer knows best what pressure is correct for thier specific design. The OEM spec is for the OEM tire spec'd for the car.

Misplacedyankee

Link and text is below...it was from an offroader's website, but offered an interesting analysis. At the end of the day it is a multitude of factors really and if you wanted to go crazy with just the right psi you would constantly be adjusting your psi for load and temperature. Based on what I read you shouldn't just automatically adjust your psi due to a higher load rating. I didn't for my 51 psi tires - never found evidence in many searches telling me to do so.

----------------------------

Every vehicle should have a manufacturer recommended tire inflation value, usually on a sticker on the driver's side door jamb. This figure is determined by the manufacturer based upon the vehicles stock weight distribution, wheel and tire size. This is probably the best value to use if it applies. However, if you have changed wheels, tires, or weight significantly, this number may not be appropriate.
Tires, too, come with manufacturer-specified inflation specifications. These, however, are not vehicle specific, but rather refer to the maximum inflation pressure the tire can handle in relation to its maximum load carrying capacity. For example, assume you have a light truck tire with a 2500 pound maximum load rating at 50 PSI air pressure. Lets say there are four of these tires mounted on a 5000 pound vehicle (with 50/50 weight distribution), so the per-tire load is 1250 pounds (5000/4). Clearly, the tire is nowhere near its maximum load, in fact it is at 1/2 load in this case. A case could be made for inflating the tire to 1/2 its maximum pressure (25 PSI in this case) based upon the load on the tire.

Actually, while there is a fairly linear relationship between a tire's inflation pressure and its load carrying capacity, it is simply not a straight line from 0 to the maximum load. I did a least-squares-fit analysis on some pressure vs. load data for a series of agricultural tires and found that the following factors seem to fit the data quite well:

mL = maximum tire Load (lbs)
mI = maximum tire Inflation (psi)
L = the actual load on the tire (lbs)
L = 0.21*mL + (0.79*mL/mI)*inflation

In other words, at "0" psi, a tire ideally could carry 21% of its maximum load (probably not true - but useful for numerical analysis) and the other 79% of its load capacity is linearly related to its internal pressure. So, from the example above:

mL = 2500
mI =50
L = 1250; solve for inflation = (1250 - (0.21*2500)) / (0.79*2500/50) = (1250-525)/39.5 = 18.3psi

So, 18 is clearly less than 25 (that was obtained with a linear interpolation), the "correct" answer is probably somewhere in between. I can say that I have run extended periods on these tires at 18 psi on pavement at highway speeds without any adverse affects. In fact, before going through these calculations, I had settled on 18psi as my air-down pressure for off-roading where significant periods of high-speed/pavement driving was anticipated. This pressure allowed decent off-road traction and let me safely cover paved sections without stopping to air-up. So I sort of see this number as a minimum safe on-road [pressure for extended driving (for these tires on this vehicle, YMMV.
Another school of thought is that you should inflate the tire such that it has uniform tread contact with the road. This can be determined in a number of ways. The easiest is to try to slide a thin card under the edge of the tread. Inflate the tire until you can just get the card under the edge a little bit. A more involved check is to place a chalk line across the tread face, drive a short distance straight ahead on a smooth surface and then observe the chalk line. You are looking for it to be evenly worn off the tread. Another variation is to measure the length of the contact patch and make it even front and rear. This works well on vehicle where the rear load can vary, such as a pickup and especially if a recommended pressure is known for the front end. Slip a paper sheet under the tire to stop at the leading and trailing edge of the contact patch, measuse the separation of the two sheets (making sure they are parallel). Then set the rear pressure such that the length of its contact patch is the same as the front.
One of the most accurate (and complicated) methods is to measure tread temperatures right after a high speed run. Even temps. across the tread indicate proper inflation. This is how race teams judge tire pressure in their vehicles. A less complicated version of this temperature-base technique is to select a cold tire pressure such that after 15-30 minutes of high speed driving results in a pressure (or temperature) rise of less than 10%. Increased temperature of the air in the tire is the cause of the pressure rise, and a rise in pressure of 3 psi is about 10% of a typical tire inflation pressure (~30 psi) and represents about a 50°F temperature rise. Note that this represents about a 10% temperature change on a absolute scale, noting that absolute 0 is -459°F, so at an air temperature of 41°F, the absolute temperature is 500°F above absolute 0. Thus a temperature change of 50°F is a 10% increase, from 500°F above absolute 0 to 550°F above absolute 0.

Often when you have new tires installed, you'll find that the tire shop has inflated them to their maximum rated pressure. Depending on the vehicle, wheel and tire, this may be good or bad. On a recent set of tires, I decided to conduct an experiment by collecting tread wear data over the life of the tires and I changed the inflation pressure about 1/2 way through the tread life. Here is some data I collected over the life of my Goodyear Invicta GLR tires (175-70R13 mounted on 13x5 rims). I measured tread depths at the inner, middle and outer tread grooves, at each tire rotation, taking the average of all 4 tires:

Tread Wear Analysis Odometer Miles Pressure Outer Middle Inner Wear/Edge Wear/Middle
232790 0 32 0.22" 0.27" 0.22" 0.000" 0.000"
237750 4960 32 0.19" 0.24" 0.18" 0.040" 0.030"
242304 9514 32 0.15" 0.21" 0.14" 0.075" 0.060"
243000? 10,000? 44 ?" ?" ?" ? ?
256988 24198 44 0.08" 0.16" 0.06" 0.150" 0.110"

So, looking at the above data, it would appear, that even running these tires at their maximum inflation pressure, the edges seem to wear faster then the middle. The tires in this study were rated at 1036 lbs maximum load, I estimate the VW pickup they are mounted on to weigh approx. 2500 lbs, so I am well under the maximum load on the tires. They are mounted on the tire mfg's design width rims, the section width of the tire is 2" over the rim width. Perhaps different tire and rim widths as well as different loading characteristics would change the results. I do drive these tires pretty hard and it looks like I have worn them out in about 25k-30k miles, but what can you expect from an OEM tire? Also, they were used (unknown mileage) when I bought them. They are only rated at 260-treadwear, so I'm not surprised. One interesting item to note, is that normal passenger car tires (standard load) are rated at 36psi and maximum inflations above that value are for meeting special vehicle requirements. There are also "extra load" passenger car tires for higher load capacities, that allow higher maximum inflation pressures (44 psi in my case).
So, answering the question of what is the "correct" tire pressure is not that easy. There are several DIY tests that you can try:

1. The "Business Card Test": On a smooth, hard surface, try inserting a business card between the tire and the pavement. If it goes in less than about 3mm-1/8", the the tire may be under-inflated, if it goes in more than about 6mm-1/4", it may be overinflated.

2. The "Chalk Line Test": Draw a heavy chalk line across al the trear faces. drive slowly forward in a straight line for a few revolutions of the tire. Get out and observe the wear pattern of the chalk. If it has worn away evenly, then the inflation is correct. If either the edge or center of the line is worn first, then the tire is under or over inflated, respectively.

3. The "Water Puddle Test": Similar to test #2, but drive through a puddle of water in a straight line, then get out and observe the wet tire tracks and see if the wet imprint is even, especially as the track starts to dry out after a few revolutions.

4. Heat is the #1 enemy of high-speed tires. The flexing of the tire's sidewalls as the tire rolls under load is the source of the heat. Higher inflation pressures mean less flexing of the sidewall and therefore less heat. Another test for proper inflation pressure is to measure the tire pressure when cold then again after 15 minutes at highway speed. If the pressure rise due to the temperature rise is more than about 3 psi, then the tire may be under-inflated.
And some tire manufacturers recommend maintaining a minimum of 25 psi in on-road tires for adequate bead retention in cornering. Often, you can contact the tire manufacturer and they can supply inflation data for your vehicle/tire combination.

Troy H

Good reading, thx!