QuestionQUESTION: I know there is a sticker on my vehicle listing the tire pressure for the stock size of tires but I have installed lower profile tires on larger rims. Likewise the max pressure listing on the side of the tire is from the manufacturer who has no idea what vehicle their tire will go on. I assume that the max pressure listed on the side of the low profile tire is higher because there is less sidewall to hold the tire up so the vehicle sticker is not applicable for this tire. I was always taught to go by the max pressure on the side of the tire (minus 4 lbs) as a target. Is this a good practice still? People running 25 series tires can't possibly be running the 32PSI car manufacturer pressure on bumpy roads or I think I should open up a rim store for their repeat business!
ANSWER: Johnny,
No, the max pressure on the tire is just that and has no relationship to what the proper pressure should be.
If you would like to know what the proper pressure should be, then you need to know 5 things:
1) The tire size listed on the vehicle tire placard.
2) The pressure listed on the vehicle tire placard.
3) The tire size currently on the vehocle
4&5) The load vs pressure relationship for both tire sizes (commonly known as the load tables)
The calculation is pretty easy to do once you have all those things, but items 4&5 are not published on the internet since the folks that set those things up (Tire Standardizing Organizations) do not have any income source other than the publication of the books that contain those tables and forbid such a publication.
But I happen to have those tables and am willing to do the calculation provided you provide items 1 thru 3!
---------- FOLLOW-UP ----------
QUESTION: Thanks for figuring this out for me. I have two vehicles in question. The first is my F-150:
1. P275/60R17
2. 30F 32R
3. 295/45R20
4&5. ??
The other is my wife's Eclipse. I just got new tires for it (Nokian WRG2) that are the same size as the original ones but have a much higher max pressure listed on them which got this whole question started. So, the fact that these new tires have a higher max pressure has no bearing on what pressure they should be run at.
1. P215/50R17
2. 32F 29R
3. same
4&5. ?? Still I should run 32F 29R then?
Many thanks!
AnswerJohnny,
The shortcut to this process is the Load Index.
A P275/60R17 SL has a LI of 110. A P295/45R20 also has a LI of 110. Therefore the pressure vs load relationship is the same - and so the proper pressure doesn't change as well.
In the case of your wife's Eclipse: So long as the tire size is the same, then the load vs pressure relationship is the same - regardless of what the maximum pressure listed on the sidewall says.
One little quirk is that there are tires with a higher load carrying capacity, but at a higher pressure. These are called Extra Load (XL) and their maximum load carrying capacity peaks at 41 psi. Most passenger car tires are Standard Load (SL) and their maximum load carrying capacity peaks at 35 psi. The load vs pressure relationship is the same between SL and XL, but the SL stops at a lower pressure. So be aware of this when comparing tires. Tires with the same LI might be SL vs XL.
Another quirk is that using ISO units (metric), the pressure (and load) doesn't translate into even numbers in both systems, so the tire standardizing bodies use a number rounding convention for their particular standards - and that creates a situation where the max load pressure is slightly different. In the case of ISO units, that's 36 psi for SL tires. That also means the load carrying capacity is also slightly different. But the operative word is "slightly" For practical purposes, it's the same.
I go into this in more detail here:
http://www.geocities.com/barrystiretech/loadtables.html
Now let me explore a question you asked originally: ".....People running 25 series tires can't possibly be running the 32PSI car manufacturer pressure on bumpy roads ........"
What sometimes gets lost is that the primary function of a tire is to carry the load of the vehicle. Put on a tire without enough load carrying capacity and it will fail - very dangerous!
This is true of all types of tires - aircraft, truck, mining equipment, bicycles, etc. Fundamentally, the tires are the same, but each type is tailored to operate in the particular application it is intended for. This is where the tire standardizing bodies play a role. They set the load vs pressure relationship for various tire sizes within a particular application. To help clarify what tire is intended for what service conditions, the tire standardizing bodies tend to chose different ways of expressing the tire size.
For example, passenger cars tend to use a size configuration that looks like this: Width/aspect ratio/tire construction/rim diameter - which comes out looking like this: P275/60R17.
But this size could also be expressed by using the overall diameter: 30X11.0R17 - EXCEPT this form is used for flotation sizes used in light truck application and those are designed to perform in slightly different ways. So it's important to recognize that not only does the size indicate the overall physical dimensions, but also the intended service.
Taking this a step further, if we confine our discussion to passenger car tires and explore the issue of aspect ratio, it is clear that lower aspect ratios tend to ride harsher (less sidewall) - which means they are intended for smoother road surfaces. Because of the shorter sidewalls, they are also more prone to impact damage (and as you pointed out, rim damage).
Clearly any given tire is going to be able to absorb a certain amount of impact force before it bottoms out and transmits that force directly to the rim - and, of course, a lower aspect ratio would transmit that force sooner. So careful consideration of the road surfaces likely to be encountered becomes important when selecting a tire with a lower aspect ratio.
Needless to say, a 25 series tire is designed for smooth road surfaces without pot holes. However, from a load carrying capacity, a 25 series tire COULD have the same load vs pressure relationship as a 75 series tire - and therefore, the same recommended pressure.
