Do you run factory PSI on your tires?

[Lee]

Well, I think we all understand that the inflation target is whatever pressure is requîred to achieve even pressure/contact across the entire tread width. That should, in theory, realize the maximum traction and minimum tire wear (I'll hold back judgement on rolling resistance, 'cause I'll bet gas mileage only gets better as pressure increases). But the only way I can think of to really judge an even contact patch, is something member Solid raised years ago in jest: infrared imaging. Might have to try and see if a cheap infrared thermometer would tell anything.

[Dan Szwarc]

Cheap IR cameras do not exist.

[TonyC]

The listed psi for the car is based on the characteristics and tire size that was originally intended to be on the car. As long as you're not changing that it's safe to follow that pressure. Tire manufacturers don't know the size or weight of the car the tire is going on so they can't provide any number to follow. Unless it's low profile or some performance tire stock pressure is almost always listed around 32-35 psi so it's safe to stay around there.

Well, let's be honest, who in his right mind would be running OE-spec, bias-ply tires on any car nowadays?

Plus, I wouldn't say that tire manufacturers are clueless of the specific cars. They have to know the dimensions and the load ratings their products need to service; that's all that matters, the other car specs being academic. They also have mandatory standards to meet; so unless they are unscrupulous (which is possible, don't get me wrong), their products are expected to serve the vehicles they are meant to serve.

And I have to disagree with the arbitrary psi figure. There is no authority that establishes that, it's just a "custom" dating back to early radials, which I do remember being made for that max cold psi amount (I remember that number being molded on the walls of all 225 or 235 75R15 radials until about 2001, when the numbers began to increase). But as technology progressed, the psi needs increased, even for tires made in the same dimensions. The standard reference is whatever is molded onto the tire wall, which is relevant because it specifies for that particular brand and model of tire, which is where it matters. My current set states a max cold pressure of 50 psi, so 32 would be dragging like running on a flat. I keep mine at just above 40, which ought to be a good balance between traction and fuel-economy.

What also matters is load rating, which is also molded to the tire wall; so I maintain great skepticism about the integrity of all these over-popularized wagon-wheels and their tiny rubber linings, especially where cars as massive as ours are concerned. I hearken back to observation of a guy I knew 23 years ago, who had a '73 Mark IV and was always doing goofy stuff to it. He once went with 205/70R15s to "lower the profile"; because of the mass of the car, which by that year nearly equalled any non-'vert clap-door, those tires self-destructed just a few months after he got them. He never considered load rating, only overall tire size, and paid for it...which is what I suspect will happen with every wagon-wheel conversion, unless said wheels are constructed with that load in mind (though possible, doubtful).

On a side note, it's good this topic came back up. It prompted me to check my tire pressures yesterday, the first such check since at least last October; and sure enough, all four of them (yes, including the new ones) were at or around 32 psi, which has to at least partially explain why my fuel-economy still isn't up to snuff, though it is significantly better since the carburetor swap (also much cleaner exhaust-wise). I got all of them to about 41 psi. I just wish I had thought of that a couple days earlier, before the trip to my doctor appointment in Topeka, just to see what effect that would have had on the fuel-economy. Maybe, had I thought of tweaking them in Texas, that too would have made a difference in my gas bill.

---Tony

[Mike]

The listed psi for the car is based on the characteristics and tire size that was originally intended to be on the car. As long as you're not changing that it's safe to follow that pressure. Tire manufacturers don't know the size or weight of the car the tire is going on so they can't provide any number to follow. Unless it's low profile or some performance tire stock pressure is almost always listed around 32-35 psi so it's safe to stay around there.

Well, let's be honest, who in his right mind would be running OE-spec, bias-ply tires on any car nowadays?

Plus, I wouldn't say that tire manufacturers are clueless of the specific cars. They have to know the dimensions and the load ratings their products need to service; that's all that matters, the other car specs being academic. They also have mandatory standards to meet; so unless they are unscrupulous (which is possible, don't get me wrong), their products are expected to serve the vehicles they are meant to serve.

And I have to disagree with the arbitrary psi figure. There is no authority that establishes that, it's just a "custom" dating back to early radials, which I do remember being made for that max cold psi amount (I remember that number being molded on the walls of all 225 or 235 75R15 radials until about 2001, when the numbers began to increase). But as technology progressed, the psi needs increased, even for tires made in the same dimensions. The standard reference is whatever is molded onto the tire wall, which is relevant because it specifies for that particular brand and model of tire, which is where it matters. My current set states a max cold pressure of 50 psi, so 32 would be dragging like running on a flat. I keep mine at just above 40, which ought to be a good balance between traction and fuel-economy.

What also matters is load rating, which is also molded to the tire wall; so I maintain great skepticism about the integrity of all these over-popularized wagon-wheels and their tiny rubber linings, especially where cars as massive as ours are concerned. I hearken back to observation of a guy I knew 23 years ago, who had a '73 Mark IV and was always doing goofy stuff to it. He once went with 205/70R15s to "lower the profile"; because of the mass of the car, which by that year nearly equalled any non-'vert clap-door, those tires self-destructed just a few months after he got them. He never considered load rating, only overall tire size, and paid for it...which is what I suspect will happen with every wagon-wheel conversion, unless said wheels are constructed with that load in mind (though possible, doubtful).

On a side note, it's good this topic came back up. It prompted me to check my tire pressures yesterday, the first such check since at least last October; and sure enough, all four of them (yes, including the new ones) were at or around 32 psi, which has to at least partially explain why my fuel-economy still isn't up to snuff, though it is significantly better since the carburetor swap (also much cleaner exhaust-wise). I got all of them to about 41 psi. I just wish I had thought of that a couple days earlier, before the trip to my doctor appointment in Topeka, just to see what effect that would have had on the fuel-economy. Maybe, had I thought of tweaking them in Texas, that too would have made a difference in my gas bill.

---Tony

Lots of oem tires note max pressure at 50psi range yet the car says 32-35 and they run perfectly fine at that. You just said it "their products are expected to serve the vehicles they are meant to serve" and its all based on size and load of the vehicle. Sure tire manufacturers have to know the dimensions and the load ratings their products need to service but you can have the same tire specified for two completely different sizes of vehicles with the only difference being what the tire pressure is noted by the manufacturer. Some cars even have the same tire on all 4 corners with different pressures noted front and back. The max tire pressure listed on the sidewall is just that- the max pressure the tire was designed for.

And like I said sometime you get tires that don't have as solid side wall and they don't feel right or wear properly and more air doesn't hurt so if there's a need for more go for it.

[frasern]

I have a low opinion of todays tires, but I'll try not to rant.
Tires have changed, even since this thread was started, they are always changing. Manufacturers are trying to balance ride, noise, lifespan, and environmental concerns, as well as changing consumer desires. In particular, the low profile sidewalls need to be kept stiffer, and so more air pressure is used.
It has always been my choice to err on the higher side, lower pressure, even slightly, increases the flex, which can fatigue the casing, todays casings can't take the abuse they did years ago and tires can fail with lots of tread left. That's my opinion anyway.

[rick]

I run 36 pounds cold in everything. Always have. Just always felt the cars I drove handled better 4 pounds over.

I think the fact that I drove motorcycles for years probably influenced my thinking. If you want to risk your life, go ride bikes on under-inflated tires.

I know what you're thinking, over-inflated motorcycle tires are dangerous too....... but cornering bikes with under-inflated tires will get you.

[Dan Szwarc]

Lots of oem tires note max pressure at 50psi range yet the car says 32-35 and they run perfectly fine at that.

Please note that MAX tire pressure is not the same as recommended tire pressure. DO NOT EVER run any tires at MAX tire pressure unless you have a desire for exploding tires.

Tire pressures are selected for a compromise between ride comfort, tire wear, tire safety, etc. There is no recommended setting for our classics so it’s left up to is.

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[Lee]

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You guys with 15” wheels can still get a full set of proper size tires for about the price of ONE 14”, so I’m rather sensitive to the subject of tire wear. Overinflation can certainly increase gas mileage, but then you risk having your tire appear in a rogue’s gallery:

[RMAENV]

I believe the tires on my vert now say max 35 PSI on the sidewall. Wear is not much of an issue as i do so few miles. I run about 3 pounds under max or 32 PSI. I really don't have any handling, cornering, ride, highway hum, or squealing issues with the 3 under max formula.

[1Bad55Chevy]

Lots of oem tires note max pressure at 50psi range yet the car says 32-35 and they run perfectly fine at that.

Please note that MAX tire pressure is not the same as recommended tire pressure. DO NOT EVER run any tires at MAX tire pressure unless you have a desire for exploding tires.

Unless you are running nitrogen filled... ..... sorry I had to be that guy!

Note I have never ran nitrogen in anything nor will I ever. But nitrogen is not affected by temperature changes so the pressure is constant regardless of the tire temp.

[kingraph2]

I believe nitrogen, being a gas, still follows Charles law concerning temperature, volume and pressure.

[Dan Szwarc]

But nitrogen is not affected by temperature changes so the pressure is constant regardless of the tire temp.

This is not true. PV=nRT applies to all gasses (as said above).

On this forum, we obey the laws of physics!

I wonder what other lies the nitrogen pushers are saying?

[Lee]

The real N2 advantage (at least concerning inflation pressure stability) is that it doesn’t contain water vapor like atmospheric air does, and therefore pressure increase with increasing temperature is reduced.

[TonyC]

Lots of oem tires note max pressure at 50psi range yet the car says 32-35 and they run perfectly fine at that. You just said it "their products are expected to serve the vehicles they are meant to serve" and its all based on size and load of the vehicle. Sure tire manufacturers have to know the dimensions and the load ratings their products need to service but you can have the same tire specified for two completely different sizes of vehicles with the only difference being what the tire pressure is noted by the manufacturer. Some cars even have the same tire on all 4 corners with different pressures noted front and back. The max tire pressure listed on the sidewall is just that- the max pressure the tire was designed for.

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

[1Bad55Chevy]

But nitrogen is not affected by temperature changes so the pressure is constant regardless of the tire temp.

This is not true. PV=nRT applies to all gasses (as said above).

On this forum, we obey the laws of physics!

I wonder what other lies the nitrogen pushers are saying?

Man yall guys get REALLY technical around here...

Nitrogen pressure change 1Psi per 10* F. This is why they use Nitrogen in race applications such as F1, NASCAR, etc.