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Proper tire pressure

True as far as it goes, but the “recommended” pressure is not based on tyre wear, but for safety. Without doing some serious test track testing, you are not going to know how you car handles at different tyre pressures when you are really in trouble, until it is too late. It is best to pay attention to the “recommended” inflation.

I think where the confusion is coming from here is the assumption that tire pressure is an exact science and either the vehicle manufacturer or the tire manufacturer must be able to put a specific quantifiable number on tire pressure and that be it, no questions asked.

The fact is there are way too many variables for that to be possible. Yes, it’s good to know that your vehicle needs about X number of pounds of air in the tires. It’s also good to know that you shouldn’t attempt to put 80 pounds of air in a 4 ply tire.

There’s cold pressure and hot pressure. The pressure listed on the tire and door is cold pressure not hot pressure. If you drive 5 miles to the service station to check your air, you aren’t checking cold pressure. Different climates, seasons, road surfaces, altitudes, speeds, loads, curves, etc are all going to make a difference in what serves you best.

Tires serve several purposes on a vehicle. First they provide traction to keep you moving and turning they way you want to. Secondly, they act as part of the vehicle’s suspension. Yes, the sidewall of a tire is the same as another spring, shock or strut on the vehicle, and unlike a shock or spring or strut, a tire is adjustable by varying the air pressure. You can also change a vehicle’s ground clearance and change the gear ratio by changing tire sizes. Race teams spend millions on tire engineers, tire testing, tire diagnostic equipment etc. Why? Getting it right makes a big difference in performance.

In the case of trucks, you can select tires based on the planned use. Papaw’s go to Hardees for coffee in the morning pickup will do just fine on a 4 ply passenger car tire. A truck used to tow and haul medium loads may need a 6 or 8 ply tire while a heavier duty pickup will likely need between an 8 and 12 ply tire.

A truck being used on a job site is going to need a heavier tire to help prevent punctures from errant nails, screws, etc not to mention be able to dig through the mud when the occasion arises. One towing heavy loads on a highway is going to need a stiff sidewalled highway treaded tire.

Race teams know that the tire will heat up increasing the air pressure on the track. The same thing happens on the road. Depending on climate or season or load or road type, heat may build more or less building pressure more or less. In the winter I tend to run a higher cold pressure than in the summer to compensate for that difference.

In flat country, you don’t need as stiff of a sidewall as you do in mountains where you are constantly turning and needing the extra stiffness in the corners. I live in the mountains so I run a little higher pressure to help out with body roll going around corners.

You can change your final gear ratio with tires. How? Most vehicles can accommodate different sizes. For instance, my Dodge is a little low geared for my use. When the factory shoes wear out, I plan to put on tires that are an inch to an inch and a half taller. This should cut the rpm’s on the engine down at highway speed and save a little fuel. Conversely, you can gear one down by putting a shorter tire on it giving the vehicle more torque.

4 ply tires generally have a max pressure of 35 psi, 6 ply goes up to 45 or so, 8 and 10 ply can handle between 60 and 80 psi. They rate tires today in letters. A, B, C, D, and E rated tires. A being a 4 ply and it goes up from there. Most boat and rv trailers have E rated tires on them and require minimum of 50 psi cold. You’d be surprised at how many people are towing heavy boats around on tires with 35 psi in them because the dummy at the Sears tire shop didn’t know come here from sickum, or for that matter have the wrong load rated tire installed on the trailer.

The guy that said find a tire shop that has the owner running it and 1 or 2 bays is correct. Sears is probably still the #1 tire retailer in America, they were when I worked there. People come in there every day to get tires and put their trust in the bottom of the barrel mechanic Sears can get away with hiring. The 2 lowest jobs in the shop are battery and tire changers. Trust me when I say this, if a mechanic is worth 2 hoots, he won’t be a Sears tire changer at minimum wage + a couple dollars for long. A mechanic running his own shop and staying in business for decades didn’t do that with the level of expertise box stores use for their tire changers.

The pressure listed on the vehicle door and owners manual is for the factory tire and is for typical conditions. Your driving conditions may vary, and likely your factory tires won’t last long before you change to some other brand, size, type etc. That pressure is a good starting point, but certainly not the final answer. The pressure on the tire is the max that tire is rated for.

If you notice your tires are wallowing around in the road, they are under inflated, it matters not what the door panel says. If you notice your tires bouncing too hard on the road, they are over inflated. If your load is mashing them down creating a larger than normal radial bulge, they are under inflated. It shouldn’t take 8000 miles and a tread guage to figure that out. If you like I do and air up for a load, over time, your tires will become looser and more dependent on the extra air in them and you may notice you can’t drop the pressure back like you could previously. While much stronger than a ballon, tires aren’t much different, they do stretch over time and the sidewalls become weaker requiring more air to keep them performing like you need.

As to the original question 3 pounds one way or the other probably doesn’t make that much difference, and being a 10% error may in fact be the difference between 2 guages and not a 100% accurate reading. A poor seal between the guage and valve stem can easily cause a few pounds error in the reading.

Skip

A couple of factoids:

Steering tires tend to wear the shoulders more rapidly than the centers. As the forces on the tire build to move the front of the vehicle, the outside shoulder tends to tuck in and the inside shoulder tends to lift.

The drive tires tend to wear more rapidly in the centers. I don’t have a good explanation for this, I think you can see this for yourself if you own a RWD.

BUT, if you own a FWD, the front tires do both, so they tend to wear evenly across the tread face (assuming the alignment is good and you’re not driving around corners like a race car driver)

So it’s not a good idea to use wear as the only criteria for pressure without considering this difference.

Also, the car manufacturer spends a lot of time and effort testing their vehicles using the pressure specified on the placard. They set the spring rates of the springs, the damping of the shocks, and the size of the sway bars based on that pressure. Changing to a different tire only slightly changes the spring rate of the tire - most of the spring rate is caused by the pressure. So unless you do a lot of testing, and are as skilled as the test drivers at the car manufacturers, I would suggest you defer to their expertise.

On a more personal note: I think that anyone who doesn’t consult the vehicle’s owners manual for advice shouldn’t be dispensing it.

On a more personal note: I think that anyone who doesn’t consult the vehicle’s owners manual for advice shouldn’t be dispensing it.

Did you consult your computer’s 400 page manual before you used it or did you defer to your past experience with computers and go on from there?

Seriously, 99% of the people don’t read manuals on things they’ve operated for years just because they get a new one. Every so often I get a new tv, they all pretty much work the same. There’s a remote with numbers on it, up and down channel and volume, plug for the wall and plug for the cable. Am I going to read the 200 page manual that came with the tv? No, don’t need to, it’s a waste of time.

I might look at a truck’s manual the first time I change oil in it to find out how many quarts of oil it holds, what the filter part number is, or to get the fuse box diagram, but other than that, I’ve never opened one, and never needed to except that time I got the lemon from Ford and needed a phone number for them.

I know how to air my tires up. I’ve probably mounted a couple hundred sets in my lifetime. I’ve owned, driven, and used vehicles for over 25 years, and don’t need a manual to figure out how to air up a tire and to what pressure. In 25 years, I’ve learned by trial and error and know what works and what doesn’t. For what it’s worth, my grandpa drove coal trucks for 60 years and from the time I was knee high I rode around with him. 60 years of knowing how to set up a 40,000 GVW truck to haul 56,000 pounds of coal kinda rubbed off on me. He is the one that taught me what I needed to know about airing up tires and I kind of figure he had a PHD’s worth of real world experience in that area.

Skip

Skip said,

“…Did you consult your computer’s 400 page manual before you used it or did you defer to your past experience with computers and go on from there?..”

Exactly my point. I don’t give advice about computers because I’m not really an expert on the subject. And while I haven’t read all 400 pages, I did do a quick look to see if there was anything I thought might be noteworthy. I suggest you do the same with your vehicle owners manual. I’m going to guess there are a few new things in there regarding tires that your grandfather didn’t know about.

Like the words of Wisdom Ford and Firestone had on the subject for owners of Explorers with Wilderness Tires? I’m glad I referred to experience and grandpa’s teaching on the subject when I had one of those vehicles.

Skip

Regarding worry expressed about adverse handling changes due to increased tire pressure over the recommended pressures, we are talking about a possibly resultant 2 or 4 psi more than the original specification, something that is routinely done by people here on Car Talk as well as yours truly. This mild increase in pressure over standard is not a factor of any appreciable consequence regarding handling. A road surface, during a violent evasive manuever, that is wet, icy, sandy, bumpy, potholed or covered with leaves is far more to worry about than this slight increase of tire pressures over standard.

I for one, have routinely used 4 psi over recommended pressure in rear tires for improved highway steering stability for both front and rear drive cars. If tire center tread wear indicates, this may be cut back to 3 or 2 psi over standard.

Ford did that to compensate for a design defect. Most vehicles’ chassis are designed such that understeer begins to become evident before the vehicle reaches its critical lateral G limits, preventing sudden flipping. With the air pressures recommended by Firestone in the Explorer tires the vehicle would feel safe right up to its limits and suddenly flip. WWII army Jeeps had this same handling deficiency. Ford compensated by recommending lower pressures to induce understeer because correcting the problem was deemed too costly. The truely sad part is that Ford had discovered this deficiency in its prototype testing and could have corrected it properly before the vehicle was released for mass production. They chose not to. This all came out in the discovery phases of the trials.

Experience with truck tires is really not all that applicable to passenger tires. This will be really confusing to the average passenger car driver that knows little beyond how to put the key in and turn it on, and that you need a new air filter every time you get the oil changed (because that is what they are told at iffy lube). You can only give one (set) of tire pressures to the average tire user or you will boggle their minds. That is why the owner manuals have been dumbed down so much.

A can understand that. I think the problems with the Explorers were more than chassis though. I know the chassis on the explorer is very similar if not the same as a Ranger but the body is taller. Then they marketed the vehicle to soccer mom’s who want to drive a truck like it’s a car, and that won’t work. The other thing is they put V8’s in those things and people started towing things they should never have towed with them. I know I fished a tournament once with a guy who towed his rig with one and had to drive us home. That was the most miserable 40 miles I’ve ever driven fighting that ill handling vehicle with a 19’ bass boat behind it. I’ve seen people towing campers with them although it’s probably safer than towing a boat since they at least can use Equalizers that can’t be used on boats because of the surge brakes.

The fact that Ford refused to admit fault doesn’t surprise me. I had an 88 F150 with the first generation hydraulic clutch that Ford tach welded a plate on the inside of the firewall where the clutch cylinder bolted on to. I wore my first clutch out on it at 50,000 miles, second at just under 70 and found the problem trying to figure out why I couldn’t keep a clutch in it. At the time, Ford had manufactured a rather Micky Mouse looking piece to fix the problem. Basically it was a half moon piece that went on the inside of the firewall bolted to the cylinder to strengthen it. Since I was out of warranty, Ford wouldn’t consider giving me the part or installing it even though it was obviously made to correct a design flaw. A year later at almost 90,000 miles, I lost my 3rd clutch and decided I had to buy and install the part if that’s what it took or go broke on clutches. I went to a dealer to get the part and found they’d discontinued it and designed a second more complex part to solve the problem that required much more labor to install. Ford wouldn’t bend on installing that either. I did buy it at around $300 and installed it myself. Then there was that 6 liter diesel I owned, but that’s too long of a story.

Skip

I’m beginning to get the idea that the average person on here isn’t mechanically inclined and if it’s not in a manual, there’s no common sense available to figure it out. They would be in a world of hurt if they owned the Grey Market farm tractor I have which has all the labels on it in Japaneese and the manuals in Japaneese as well with no english translation. Fortunately, they seem to use normal numbers instead of upside down dog houses for things like PTO speed and oil weights.

Skip

Skip,

Beads and Beads is right. There are some fundamental differences between passenger car tires and truck tires. For example, look at the specs for a P245/75R16 and an LT245/75R16. They are the same “size”, but have different load carrying capacities and different maximum inflation pressures (BTW, don’t look at Tire Rack’s web site for the specs. For some reason, they don’t put the letters in front of the tire size and uinless you know what the other clues are, you won’t be able to sort out the “P”'s from the “LT”'s.)

This is where the “manual”, that you so disparage comes in. It will list some of the details for those of us who are more mechanically inclined. And of course, there are more details in more detailed manuals.

And as B&B said, a lot of folks who ask questions on this web site are looking for a simple straight forward answer - just like the OP of this thread. And the placard provides that answer in a simple a reliable way. They don’t have to know how the answer was derived - they just need to know where to look to get the answer. And that answer is on every light vehicle sold in the US - required by law. This is not something that the vehicle manufacturers take lightly - even though they may make a mistake now and then. They are, after all, populated with human beings.

But just to be fair, you did point out in your first posting that the placard is where a person should start - and I think we can all agree on that!

The sad part is common sense and innovation are what made this country what it is. Today we’ve got a country full of people who can’t figure out how to butter their corn without an instruction manual.

I was brought up to take what I had, look at the problem or job at hand and figure out how to make it work. It came from running around with grandpa from old enough to walk on up. Grandpa never had a formal education. He went to work at 13 mining coal with a pick, shovel and donkey. Somewhere along the way he became a country boy engineer. He designed coal handling equipment for schools, commercial users, and homes. He designed his truck to do what he demanded it do for his work not what Ford or some manual somewhere said it could do. Yeah, along the way he and I have both came up with things that wouldn’t work but 99% of the time we’ve managed to figure it out.

I know full well that there are major differences in tires of the same size. LT isn’t a very good indicator of what you have in a tire. Anything designed for a 1 ton and under is an LT tire. You have to look at the load rating coded on the sidewall of the tire. Is it a C, D, or E. There’s also the plys on the tread and sidewall that are closely related. There’s the DOT assigned tread wear rating on it that superceeds the manufacturer’s claimed mileage warranty, There’s max and minimum inflation numbers on the tire.

You are right, the simple straight forward answer is on the door. That said, the answer isn’t set in concrete.

Skip

That’s a good point. Would you take classes form a professor who never opened a book or a manual? I wound not. I would expect an educator, or one who dispenses advice, to avail himself of all of the available knowledge, both from books and from experience. The idea that you can be enough of an expert to dispense advice from only experience is rediculous.

Should we learn from experience? If course. Should we pass on what we have learned based on experience? Certainly. Should you ignore the owner’s manual because you think we know better? Feel free, but you better expect some push-back when you tell everyone else to do it.

One last thing. Those who don’t consult the manual that accompanies a new computer will probably either miss out on some cool features or end up paying an expert to teach them.

Would you take classes form a professor who never opened a book or a manual?

I have, and more than once over the years. The best teacher/instructor bar none I ever had was a highschool advanced biology teacher that didn’t even have a text book for his course. I’ve got an agriculture degree and have taken a lot of college biology classes. Honestly, I can’t think of one class I took in college that I didn’t think Ray had me more than well prepared for.

Over the years, I’ve seen a lot of extremely book smart people that couldn’t apply what they know because they have no comprehension of how it works in the real world.

One of the things that aggravated the heck out of dealing with Ford on that junk F250 that I had was the fact that their service techs, engineers, and everyone in the company from what I’ve found is taught to work on the truck with a computer. If the computer doesn’t say it’s broke, then it’s not broke, end of story. If the computer says it’s broke, change it, end of story. The old time mechanics would have pulled that EGR valve, noted the intake and valve were full of soot and realized that simply replacing the valve is not going to solve whatever is wrong that’s causing the soot to be stopping up the intake. However, the computer just registers that the valve isn’t functioning, it doesn’t register why it’s not functioning, and for their part, it’s not necessary to know why.

You kind of have to wonder what manual a person like Bill Gates read when he decided it was possible to build and program a personal computer? The Wright Brothers must have drove every intellectual of the time nuts trying to say that a lawnmower engine on top of a odd looking frame would make the whole works fly like a bird.

People take classes every day from “Professors” who never open a book. We have apprentice carpenters, electricians, plumbers, etc that have to work under a master for a period of time before being allowed to go out on their own. I would consider the Master Carpenter the professor for that endeavor. One of the best machinists I ever knew never finished high school yet he could design and manufacture equipment to do whatever you needed done. You can learn a lot of things well by doing them.

Skip

So there you go! Throw away all those useless books and play it by ear! What’s the worst that can heppen? What a great example of open-minded discourse!

Most of us will never find out what an emergency handling situation really is. That’s good. But for us who do, then I would like to have the tyres at the proper pressure as determined on the auto manufacturer’s test track.

I read all the manuals. You can find some very interesting things in there. I even read the manual for my new mower, cover to cover (about three pages) before using it. Yes it did have some information in it that I did not know and I have been mowing lawns for 50 years.

While few people do read the manual, most would do well to do so. Back when I sold cameras, most of the problems people had with a new camera was answered in the owner’s manual. Of course the people who had the problems never read the manual.

It is prudent to RTFM, but not everyone is prudent.

I can’t honestly believe that someone that’s mowed grass for 50 years with a lawnmower needs an instruction manual to use a lawnmower. I’m sorry, but that’s just beyond my understanding. There’s illiterate people mowing grass and doing it successfully every dang day of the year.

The next thing that’ll come out is the necessity to take 40 hours of continuing education classes in order to purchase a lawnmower or any lawnmower parts. After all, we don’t want anyone to get confused and think it’d be ok to stick your hands and feet under the mower deck while the motor is running.

Y’all have more or less been insinuating that I’m a dumb ignorant hillbilly all through this discussion, but I do know how to put air in tires. I’m sorry, I don’t need a manual and 2 know it all but can’t do it professors telling me how. If you need a manual, that’s fine by me, read it, take notes, outline it, go to lectures on it, have a manual reader’s anonymous group to discuss the merits of reading manuals. Makes no difference to me.

Somewhere in that dang lawnmower manual it will probably tell you how to install a new belt to the deck. It’ll probably tell you to adjust the tension pulley until there is approximately 1" of play in the belt when the drive is engaged. It won’t tell you how many turns to turn the dang nut, because a belt is made a hell of a lot like a tire and it stretches, what it tells you is adjust it till the tension is correct. That’s exactly what I said to start with on these dang tires. I said Start with the pressure listed on the door and adjust it as needed for your situation.

I’m glad my 1974 Massey MF14 is still mowing grass today so I don’t have to learn how to operate some new contraption that’s half the machine of the one I have.

Skip

No, you have it wrong. We all admire you. I know I do. You are far smarter than me. I can still learn things once in a while from reading or talking to someone else. Clearly, you already know all there is to know. I can only hope to get to that point one day. (My teenage son is very close to it already.)