Do you run factory PSI on your tires?

[Dan Szwarc]

This thread is closing in on the Mouse Attack thread for popularity!

[LC67Vert]

I run 34 pounds of pressure on the tires on my '67 convertible with 235-75R-15 tires.

[Mike]

I disagree. That may be true for vehicles made over the past 20-ish years. It is not true across the board for anything older. In the '90s I remember seeing only a handful of tires that stated to withstand 50 psi...and half of those were donuts. Running at 60% inflationary capacity is bound to cause tire wear as shown in nearly every source on tire wear. And it seems that everyone is conveniently ignoring a tiny detail: Max COLD pressure. That means the maximum safe air pressure the tire can be inflated to when not driven and thus not heated by friction. How many here will refute that radial tires are designed to inflate beyond that limit when friction-heated? Proof of that is in tires you see inflated to 100psi or higher by idiots and which still don't yet burst. Haven't seen that? Irrelevant; I have. 90% of max-COLD is fine. 60%, not really. And I again state, the decal and manual references for these cars are long-since obsolete, meaning they are not to be regarded for upkeep of any tire made today.

I don't expect to "win" this argument , as it's been going on almost as long as the Forum, with the same full-circle (pardon the pun) endings. Just giving my take on what I've been taught and what I've observed ever since I started driving. Somebody asked for input, I gave input; it can be accepted, it can be rejected. Won't change my standards on tire upkeep.

---Tony

I guess i'll once again disagree too. The majority of vehicles from the 80s and 90s still run around with new tires and their vehicle listed tire psi perfectly fine. No abnormal wear and the tires look the way they're supposed to, not under inflated. The decals and references are still perfectly valid to go by if they're still running the same size of tire. New cars still use the same old psi listing in the 30s with the much higher max psi on the side wall so running that lower percentage of pressure clearly isn't an issue.

Sure characteristics of a pertucilar model of tire might be different enough that running the original pressure is an issue, or in the case of these cars it's out the window to begin with because who's still running bias ply tires. In those situations it's safe to start around that standard 35ish range and adjust from there if there's a handling issue or by following the wear patterns that Lee posted.

This only goes to show there isn't always a simple answer. Different people have different opinions and experiences and what might work with one person and their car may or may not be the same as someone else's.

[TonyC]

The last part I agree with: No simple answer. Not my concern, and I have no desire to make it a heated topic. I'll respect others' takes even if I don't believe in them ('course, every time I'm dismissed as wrong or crazy or stupid, I end up proven right...but I don't believe in fighting wars that aren't necessary; besides, on the other hand, if my advice is heeded, the chance of my being wrong increases...not to "every time" level, but more often than not, so it may do me good if I am dismissed).

What is a concern, however, for me is making sure these new tires, especially the left-front, don't wear out and blow on me due to delayed consequences of that "Killeen Welcome." One thing I've noticed is, if I take a right turn hard enough to dip the suspension, the tire tread will hit the caved edge of the fender. My workaround so far has been to take right turns at a tortoise's pace to mitigate that; but the real answer is to do all I can to pull that fender damage out. Yesterday I also started hearing some rolling noise from that wheel, which means I also should go deep into that wheel and inspect the bearings for damage. I didn't hear any such noises right afterward, so I let it be at the time. I think it is wise to do an inspection just in case. If worse comes to worst, at least I have a complete unused set of bearings to swap in. That can also give me a chance to check the spindle's condition; I hope I don't need another one, because I'm hundreds of miles away from prospective replacements. But, that has to wait a week, when I have money again to spend at the auto center.

---Tony

[Lee]

It appears that something called the “chalk test” is commonly used to determine correct inflation pressure when pairing non-original wheels or tires (or in our case, metric radial “equivalents” on cars never originally equipped with them). It is simple enough to perform, and at least might get us out of pure SWAG’s. The principle is sound: achieving an even contact patch that doesn’t place uneven load in the center (from overinflation) or on the sidewalls (from underinflation). I may get ambitious and try it someday…if so I’ll report back.

https://fourwheeltrends.com/what-is-the ... -pressure/

[LithiumCobalt]

Max tire pressure on the sidewall is neither ideal nor recommended. It wouldn’t have been recommended when these cars were new if they had radials at the time, either. All the max cold temp reading tells you is what is a safe max operating pressure is. There is a balance between ride comfort and fuel economy. That is where the manufacturers set tire pressure recommendations. Can you run up to the max cold pressure? Probably, but for what reason? If you think you will ever squeak more than 13-14 mpg out of these cars and that is the basis for the overinflated tires, then we are living in a dream world where reality doesn’t apply.

[kingraph2]

Just for a little clarification, having retired from tire manufacturing, Nitrogen in tires is because the size of the molecule is slightly larger than the oxygen molecule and therefore doesn't permeate as easily through the innerliner (which replaced innertubes) or the sidewalls.

[TonyC]

All the max cold temp reading tells you is what is a safe max operating pressure is

Wrong. Max operating pressure is not indicated, only max-cold pressure. There's a reason for that: Max operating pressure is factored in automatically in the tire's production; max-cold molded onto the tire factors in the increased difference of 10 to 15 psi in operating conditions. This is intrinsic to all radial tires and has been for the past two generations. For those in Rio Linda, that means you can put in 50 psi into a radial tire rated with a max-cold of 50 psi and not be afraid of a blowout, because that tire is designed to expand and handle the roughly 65 psi it will have from rolling on the highway. Only if the tire is defective from its factory would you have a blowout...and we know that can happen (e.g., Firestone).

Actually, there is one condition I just thought of that fuels my overall disagreement with the consensus of this thread, which can make a difference in the whole premise of the argument: Frequency of use. The more a car is used, the more difference that will make in tire maintenance. If a car just sits in a museum display, or is driven not even 1,000 miles a year, if that, then I can see and concede the validity of the low-pressure/factory-figure arguments. If, however, a car is one's main source of motorized conveyance, then my argument becomes more valid. Nobody can dare argue that frequency of use is not a factor, unless they are just trying to argue purely for the sake of arguing.

That said, how many here actually use their cars, a lot? How many folks here consistently rack up the average regular-transpo mileage per year (or more) on their old-timers, year after year?
How many participate in, let alone validate the existence of, the annual mileage challenges? Including myself, I can think of only one. I understand now why I'm the odd man out in this discussion.

More often than not, understanding the above scenarios, the consensus would be right. My case, however, is totally different and requires a different approach, which also makes sense why my standard works for me but not necessarily for anybody else.

---Tony

[LithiumCobalt]

Frequency of use really has nothing to do with it. I don’t run my daily drivers at max psi either. In fact, you could argue to overinflate the tires for cars that DONT do a lot of moving to avoid flat spots.

Ignoring the consensus of the group that has infinitely more years of combined experience seems a little ignorant, but you’re free to believe what you like. I guess no harm, no foul. As is a popular saying these days, “You do you”

[TonyC]

If it works, I do in fact do me...and it works, so the consensus this time is wrong where my situation is concerned. There are lots of things I have admitted over the years that the consensus was right about. This isn't one of them; in fact, I tend to recall modern philosopher E.L. Kersten's take on "tradition," as my own education on the matter, which dates back far before the Forum, doesn't sync up with popular view.

I don't overinflate; 90% of cold-max is not overinflation. And frequency of use does make a difference when that frequency is regularly five figures per year, with a car that any consensus nowadays would say is madness to use as one's sole means of motorized conveyance.

But, that's me doing me.

---Tony

[Dan Szwarc]

I pump my tires to 40 psi for winter storage to reduce the likelihood of flat spots. Drove on them this way once, on accident. What a harsh terrible ride!

[Lee]

For anybody thinking about using the “chalk test” to determine inflation pressure for an even tread footprint…don’t bother. The chalk wears off pretty much evenly wherever the tread touches the road. It can’t really discriminate against underinflation cases where the sidewalls get the majority of pressure/wear, or overinflation where the center takes the brunt. I experimented with pressures from 26 to 46psi…no visible wear differences. The only thing it MAY be useful for is determine the width of the contact patch at the sidewall (see highlited red area), but I had better results in determining that by just pushing a piece of paper under the tread as far as it would go (see 2nd pic, only about 3/8” difference from high to low). Since radial tread is rounded on the shoulder, it’s hard to say exactly how much should be in contact with the road using that technique.
Tony, if you have a tread gage, or even a calibrated penny, I’d be interested in knowing if you can see more of Abe’s head in the midline of your tires than at the shoulders. You’d be a good test case for long-term high inflation results.

[TonyC]

Lee,

Had that thought come up before Killeen, I'd gladly oblige. As it is now, with brand-new tires and new alignment, I can't really contribute anything relevant to the experiment. However, what I can contribute is my observations of tire condition over the years, right up to a month ago. As many already know, much to their disdain, I consistently keep my pressures in the 90% range of cold-max, and have kept this practice for many years; the only exception was that period in Texas last month and about two weeks afterward, when I finally thought again to check my pressures. Aside from years and kilomiles racked up, the treads on my tires never prematurely wore in the center; wear was always focused on the outer edges (obviously, alignment deviation). That outer-edge wear would be slowed down by rotating tires, but the fronts would still show wear in that spot eventually...and always only in that spot. This is why I keep insisting that I'm right and the consensus is wrong.

There was one time, my only literal blow-out experience, that a tire had exploded on me, thru its inboard sidewall. But, that was in 2005, when the car's suspension still had not yet been touched (and as a consequence was wearing out); and I had a hemorrhaging, not a leak, from the front oil seal, splashing escaping oil all over that side...and we all know what effect engine oil has on rubber. So that incident cannot really be considered relevant to the topic.

Yes, my tires would eventually thin down in the center of the footprint...but that always, every time, consistently, has been the result of thousands of miles over about three years a set. On a side note, when I went to Discount Tire for a new pair of tires after the "Killeen Welcome" (well, I actually went to see about getting only the valve stem replaced, but the tech there said the wear on the outer edges of both fronts was already past their tolerance, requiring a new pair of tires), the tech mentioned that Hankook is not exactly renown for long tread life. The fronts I have now are supposed to be better in that regard; I still have to make note of the brand and model for the Forum.

---Tony

[Solid]

If you go look it up, you will discover that the original tires for the 60's slab side cars did not actually meet the load rating that Ford recommended because there was no such tire at at the time. So... even if you set side the fact we are all running radials now, the factory pressure was probably never a good benchmark. I'd target the pressure that achieves the targeted load rating for the tire you are using.

[Lee]

I'd target the pressure that achieves the targeted load rating for the tire you are using.

Solid, I considered back-calculating the correct pressure that way, but when I looked at pressure vs. load charts for modern tires, new tires can carry SO much more load, that (using the P235/75-15 example) even 26 psi (1750 lbs capacity) would constitute overinflation:
https://tirepressure.org/p-metric-tire- ... tion-chart