Street Tire Pressures

Would it be incorrect to think that increasing pressure, perhaps all the way up to sidewall pressure (44) would help protect the sidewalls from damage on these horrifically potholed roads? I tore a hole in an almost new snow tire running at 35psi (glove box sticker says 30, for the stock 185/65R14, I'm running 175/65R14 General Altimax Artics and 195/60R14 Altimax HPs in the summer).

No, you would not be incorrect. It will probably cost you some traction.

I think I can afford some traction, as these have to be some of the grippiest tires ever in the cold/wet/snow/ice.

I tend to run my tires at whatever the book says to use for a fully loaded vehicle.

I tend to run what the sidewall of the tire says.

But the PSI listed on the sidewall of the tire is at max load.... that means almost all of the time you are running your tires overinflated. What do your wear patterns look like? I usually use the wear pattern to determine what psi I run them at.....

my general rule for dd's is 35-40 depending on vehicle, i do 35 in the wifes hyundai and 40 in my f250

I'm not worried about wear patterns(modern radials don't seem very sensitive about pressure vs. wear in my limited experience), I'm worried about losing $80 every time one of these comes my way.

Potholes.

Thanks Obama!

I run my studded Arctics on both cars at 5psi over the door pressure, but that's mostly for control when I get back to dry roads.

I've always heard from off-roaders that running lower pressures helps reduce the risk of puncture, but that also really reduces your handling in all other regards. We need tweels.

I would think running a tire at max pressure would mean that when hitting a pot hole the pressure in the tire, when it is compressed, would go way beyond the normal pressure, possibly increasing the chance of a blow out.

Depends why they are blowing out, but if it is pressure related and not physical damage, I think you are going in the wrong direction. Of course, you go to low, you will start hitting the rim.

I think its pinching or hitting the rim internally from a bottom out that killed mine(hole in sidewall near shoulder, I was HOPING it was a blown bead when it happened. I still haven't gotten it swapped to inspect.

In reply to airwerks:

Anecdotal story, take it as you will.

In 30 years of driving, I have never worn the center of a tread faster than the edges. I have also never run the car manufacturer's recommendations for tire pressure. I drive fairly hard and usually eat the outside edges first.

The BMW - 40psi

The work van - 50psi The last set on this went 98K.

F350 - 70

The only exception is the Samurai. The tires call for 65 and I run 21. They are a C range tire rated at 3400 pounds each and I have them on a 2500 pound toy. Even at 21psi, they don't bulge the sidewall. Off road, I will probably run 8-10. It cleans the tread and makes them more puncture resistant.

One thing to consider about car manufacturer recommendations is what they want. I know there was some speculation that Ford recommended a lower pressure on the Explorers to limit traction during a slide. Low pressure let the carcass deform and and limited traction so the Explorer wouldn't flip. It also made it less twichy and gave a comfortable ride. I think they were recommending 26psi on a LT tire designed for 50+. There is some debate as to if that caused the Firestone blowouts rather than a tire problem. Car manufacturers have other considerations when recommending pressures.

My only consideration is tire life. Hard tires run cooler and are less likely to have a heat related failure. They deform less during cornering and the edge tread lasts longer. They are less likely to get pinched in potholes and on curbs and the protect the wheel from damage better.

My 2 cents and it's worth exactly that.

I run all my tires at max inflation and I never get blow outs or tire wear other than perfectly flat.

Max inflation is a terrible idea in all aspects of vehicle dynamics. I'm actually quite shocked to hear that given how savvy I know you all to be with all things car.

Overinflation won't protect you from pothole blowouts, in fact it can do the opposite. During high impacts, the internal pressure spikes upward. They are designed with this in mind when they put the max pressure on the sidewall, but the risk of blowout is much higher if you are at max pressure.

overinflation also dramatically reduces traction, wears the inside of the tread faster (doesn't matter if its radial or bias) makes ride quality worse, removes progressivity from grip/slip, can cause upsets on bumps during cornering and braking.... its a terrible thing.

Use the inflation recommendations on the doorjamb sticker as a baseline. You can adjust up and down, but the bottom line is this: The sidewalls do absolutely nothing to suspend the weight of the vehicle. The only thing they do is hold air pressure in the tire. Air pressure is what suspends the weight. This is why they put that number on the sticker. My factory tires were 255/50-17. I switched to 245/40-20. That is a massive change, but it did not require any more or less pressure to suspend the weight of the vehicle.

Here's where it gets tricky. The max inflation pressure corresponds to its maximum weight capacity. If you put some 215/75-15s on a small car, you inflate them to the recommended 28 psi. If you put those same tires on a large SUV, it requires 45psi to suspend the truck which is greater than its (for example) 44psi max rating. The max inflation pressure is strictly that... the most air you can put in it within its pressure-holding capabilities. It has NOTHING to do with proper inflation. The same tire could be standard size fitment on a 2000-lb vehicle or a 5000 lbs vehicle. Simply inflating it to max psi is completely counterintuitive. Using max psi is like buying adjustable shocks and just putting them on the stiffest setting.

Truck tires are constructed differently. E-range tires (sometimes denoted by a 10 ply rating) are designed with thicker, more durable sidewalls, but again... that has nothing to do with weight capacity. It simply has to do with how much pressure they can hold and therefore how much weight they can handle. A radial P-metric tire deflects the tread much more with psi, so over and underinflation can cause a big difference in treadwear on the insides or outsides of the treadwidth. Truck tires (E-range and up, floatation, etc) are constructed so stiffly that the majority of flexing with psi happens in the sidewalls and not the tread. That is for a reason. A car usually has a tank of gas and a couple passengers. A truck can vary greatly from empty weight to full payload.

My point is; why on earth would anyone use the sidewall max psi? It pays absolutely no dividends. It could be just downright dangerous.

In reply to curtis73: Running at the tire max pressure can result in higher fuel economy. Testing will tell on you vehicle. The tires I run have stiff sidewalls and handle hard cornering at max pressure better than at factory pressure. I will be running some softer tires this summer and I will have to try different pressures to see what is best.

On my last summer tires I could tell when pressures fell buy the slight loss in handling.

With all due respect, I completely disagree with Curtis. If you put the exact same tires on the car as new and want the car to handle exactly the way the car company wanted it to handle than the door sticker is relevant. In every other instance the optimum pressure will most likely be different.

If you put wider tires on the car and increase the size of the tire contact patch than the force applied to each square inch of surface the tire touches is different. Different friction properties in different tires mean even if everything else stays the same than each square inch of surface area has different amounts of grip. Stiffer or softer sidewalls mean more or less deflection of the contact patch with respect to the rim and more or less distance between the road and rim over a bump. Wheel offset changes affect scrub radius and steering force and feel. Combine several of those factors at once and it's all out the window.

Or maybe the car just squirms around too much at door sticker pressures.

For me, the max load pressure is usually closer to where I want my tires to be and I never even look at door stickers anymore.

In reply to curtis73:

Once again, a anecdotal story.

At door pressures, the front are eating the sidewalls on just about everything I own.

Case in point. These are on my work van. These tires have about 40K on them. Door pressure recommended is 41psi. Tire recommended pressure, at load rating, is 50psi which is what I run. Wear is mostly even, with more wear noted on the outside edge.

These tires are rotated every 2nd or 3rd oil change (10K-15K miles). The rears are virtually identical.

Drop the pressures 10 psi and the wear pattern moves up the sidewall about 3/16 - 1/4 inch. Load is not only the weight of the vehicle, but also how the vehicle is driven. I guess you could say I load the tires more than most people. Consequently I run more pressure than the average that the manufacture puts on the door.

These are on a Ford van. You know, the van everyone says eats tires. It was last aligned 100K ago when the ball joints were replaced. My last set of tires went 98000 miles, not following the door pressures. My last vehicle blowout was 25 years ago from a under inflated tire. Right, wrong or indifferent, I think I will stick with what the tires recommend. It's stood me well for 30 years and millions of miles.

oldopelguy wrote: If you put wider tires on the car and increase the size of the tire contact patch than the force applied to each square inch of surface the tire touches is different.

While I agree that the sticker isn't god, it is a wise starting point.

In response to what I quoted above, this is a complete myth that has been debunked since about 1950. The contact patch (within a very small variation) is the same regardless of treadwidth. If your 185/75-15 tires put about 10 square inches on the pavement, then so will 255/50-15. Its just that the wider tires have a wide contact patch that isn't very big front to back. The skinny tires have a longer, narrower contact patch.

Even if they DID increase contact patch size, that alone wouldn't be responsible for increased lateral grip since there is no additional weight to increase coefficient of friction.

They handle better because of the wider contact patch, not a bigger contact patch. Tall skinny tires deflect and have a progressive transition from grip to slip. Wide, lower profile tires tend to have a faster progression from grip to slip.

What I'm saying is that max psi on the sidewall is a reference point for the tire, but since that tire might be on anything from a Yugo to an S10, to say that one psi (i.e. the max psi) is right for all applications of that tire is ludicrous.

pjbgravely wrote: In reply to curtis73: Running at the tire max pressure can result in higher fuel economy.

At what cost though? I will have to show you what my truck tires look like. The previous owner kept them at max pressure. The center of the tread is bald while both shoulders have 5/32". Had they been run at proper pressure, they would have 20k left in them, but now I need to replace them prematurely.

My point is not to argue, I just can't believe that people use max pressure on passenger tires. Truck tires (as I said) are different. you are doing the proper testing, gravely. If your tires appear to scuff 1/4" up the sidewall at anything less than 50 psi, then you should be running 50 psi. I kept about 60 psi in my E350 powerstroke due to the heavy diesel engine up front. Conversely, lets say you put E-range tires on your van which typically have a max PSI of 80. That doesn't mean your proper inflation pressure is now 80 psi. It will still be within a few psi of 50 since you haven't changed the weight of the van. Now, when you're hauling something heavy, you have the ability to inflate up to 80 so that the carcass doesn't sag and create heat failure.

But, the "tire recommended pressure" as you said is NOT a recommendation. It is a maximum. Proper tire inflation for the vehicle and use are what is important. I guess I'm saying this: the fact that your van's tires are properly inflated at 50 psi AND the fact that 50 psi is the max inflation does NOT mean that max pressure is correct. It just means that in your case they are the same number. Those same tires from your van when put on my 65 Scout might have proper tread contact at 35 psi.

I really mean no offense to any of you, but this is really auto 101 stuff. Since when was it acceptable to just use max inflation on a tire? Sure, an E-range truck tire is a fair trade off, but on a p-tire? Never.

Here's the thing, I know, from experience, that at 35psi, 5 over what Toyota thinks I should run, the tire will in the event of a big pothole smack the tread into the wheel hard enough to ruin the tire(hole in the shoulder). Knowing that the pressure is what holds the tread away from the wheel, I need more pressure to protect the tires.

I think I'll go with 40, or maybe 42 this time, if I hit another one and it violently explodes, then I'll back it down.

I remember reading in my 92 Camry(hey, it was free) owners manual that if you were cruising at speeds over 100mph, you should inflate to sidewall pressure, versus the 28pi or whatever it called for (at which it drove/handled like ass, much better around 35-40psi) unloaded in normal driving. Its not like bring the pressure to max is gonna open a black hole or something.

Wow, I've never heard of people running tires at sidewall pressure on purpose! I have seen plenty of people come into the dealership with 70 psi in their Altima tires though(and yes, they are bald in the center). Seems to me like even a small pothole could cause a blowout as pressures go up...it's much easier to pop a balloon that's inflated to the max than it is to pop one that is mostly flat. I'm no tire guru, but the engineers that came up with the door placard probably spent a little bit of time (maybe that's a bad assumption) figuring out the optimal pressure for that vehicle, because obviously people will put multiple different tires on that have different max pressure ratings throughout the life of the car. It seems to me that it is, at minimum, a good starting point. But again, I am no tire guru.

In reply to sethmeister4:

The given pressure on the card in a passenger car is typically as low as you can safely get away with without the tire coming apart from heat buildup, plus some degree of safety factor (unless you drove an Explorer in the 90s, then there was no safety factor, hence that problem), so it will ride nicely, or to cover up some dangerous handling flaw e.g. early Corvairs did not have snap oversteer issues with the factory recommend 15psi in the front tires.

curtis73 wrote:

pjbgravely wrote: In reply to curtis73: Running at the tire max pressure can result in higher fuel economy.

At what cost though? I will have to show you what my truck tires look like. The previous owner kept them at max pressure. The center of the tread is bald while both shoulders have 5/32". Had they been run at proper pressure, they would have 20k left in them, but now I need to replace them prematurely.

The rest of my post said that my tires wear completely flat, I can post a picture if you don't believe me. If I saw uneven wear on my tires I would adjust the pressure, my last summer tires I ran right down to the wear strips completely flat. Each car is different, each driver is different. There is no set pressure for all. Trial and error is the only way to know.

Cars come thru the shop with different brand tires on each axle or all 4 wheels. If one tire's max inflation is 44 and another 35, how can putting max pressure be right?