Car Tire Pressure - how high do you go?

[mpc]

Assuming this is for a normal size/weight passenger car...

The car maker's inflation guide on most cars is for "best ride" comfort typically, not necessarily best safety (i.e. best cornering & braking grip). And car makers don't cover the tire warranty, that's up to the tire manufacturer so the car maker doesn't much care about lower inflation pressures possibly reducing tire life. So many, if not most, door jamb recommended tire pressures are probably best viewed as a minimum pressure for the stock tires. That pressure is "safe" enough and yet gives a nice ride for the test drive at the dealership.

The tire manufacturer will place a maximum inflation pressure on the tire sidewall. Under normal circumstances there's no reason to be close to that pressure on a cold tire. Sometimes there is a minimum pressure as well but that's rare.

For most passenger cars, somewhere between 32 to 36psi is ideal. Ideally the tire tread is flat on the pavement - when viewed from directly in front of or behind the tire. As others posted earlier, too little pressure causes the "contact patch" to take on a rounded "W" shape, with the center of the tire not really in firm contact with the road... ergo the outer edges wear too rapidly and the potential grip of the tire center tread is wasted. Tires of course are only round when not supporting the vehicle; the part of the tire directly between the wheel center and road is flattened a bit - the sidewalls are flexed. As others posted, that flexing creates heat in the tire as you drive. Some flex is normal, and in fact essential, for the tire to do its job but excessive flexing (from under-inflation) overheats the tire and is a potentially dangerous situation. (Ford Explorer debacle years ago...)

Over-inflation makes the tire bulge out in the center - so the center of the tread does all the work and wears out too rapidly and again, grip is compromised because the outer edges of the contact patch aren't doing their full job.

Changing type of tire from the "OEM" tire often requires different air pressures; not all tires have the same sidewall stiffness. Tires have a "load rating" --> basically the maximum vehicle weight each tire can support. If you install a higher load rated tire (compared to the OEM tire) on a vehicle, that tire may be over-inflated at the same psi that was correct for the original tire.

Trucks, vans, etc. - work vehicles or heavier vehicles - often do require more pressure; that should be reflected on the vehicle's door jamb placard. The tire is basically a rubber damper and the air inside it is the spring to support the weight... so for a heavier vehicle you need a stiffer spring to support it ==> more psi. Or you need a much fatter tire.

Another consideration for newer cars with "Tire Pressure Monitoring Systems" (TPMS) required by law now... those typically trigger a "tire low pressure" dash alert at pressures typically in the 27 to 29 psi range (for average passenger cars). Given that tire pressures can vary up/down with air temperature changes, you may find that setting the tires to 32 psi on a warm day causes your TPMS warning to trigger on a cold morning.

mpc

if anybody cares, I run my cars at 36psi - an older Corolla and a sports coupe. at 32 psi I can see abnormal tire wear after a few thousand miles as well as detect much sloppier steering.

[LCHIEN]

Is that really true? Or rather do you you think the atmospheric effects of temperature (hot air/road) will have a greater impact via increased pressure than the internal friction of an under inflated tire? I'm not claiming to know the answer, but it seems to me the Explorer/Firestone blow-out roll-overs were largely blamed on under-inflation. I strongly suspect that those max inflation ratings are determined to be safe knowing that when driven they will increase, and that driving a tire is not an unexpected use case...

That said, I've always been told to go by what's on the door/in the manual, not what's printed on the tire itself (which seems counter intuitive but it's what I do).

Yes, running an underinflated tire will cause a lot of sidewall flex and overheat the tire as well. But for the most part we're talking severe underinflation, not the normal pressures that the manufacturer recommends.

In the case of the Ford Exploder/Firestone, I personally think it was a bit of liability for both sides... the tires were not completely up to snuff and Ford made a semi-intentional mistake in recommended pressures. Either one by itself was probably not a disaster but in combination it was fatal.

Basically the car manufacturer has an array of load ratings for the OEM tires - the load rating rates the stiffness of the sidewall so the tire won't flex too much in normal use. Since he's buying tens of thousands of tires, he can chose any value the tire manufacturer makes, not just whats in stock. The car manufacturer has to balance handling, ride quality and tire wear with the correct tire pressure rating and choice of the load rating allows him to do so given each model has a range of vehicle weights.

If your tire seller is recommending a much higher tire pressure than the OEM then you have to wonder if he's unscrupulously sold you a tire with a much lower load rating than your car needs because its what he had in stock.

[cabinetman]

Is that really true? Or rather do you you think the atmospheric effects of temperature (hot air/road) will have a greater impact via increased pressure than the internal friction of an under inflated tire? I'm not claiming to know the answer, but it seems to me the Explorer/Firestone blow-out roll-overs were largely blamed on under-inflation. I strongly suspect that those max inflation ratings are determined to be safe knowing that when driven they will increase, and that driving a tire is not an unexpected use case...

That said, I've always been told to go by what's on the door/in the manual, not what's printed on the tire itself (which seems counter intuitive but it's what I do).

Furthermore the tire rubber gets softer and weaker as the temperature goes up, more likely to burst from maximum pressure. Maximum pressure at maximum operating pressures would be the worst combination.

That statement might have been meant to read..."Furthermore the tire rubber gets softer and weaker as the temperature goes up, more likely to burst from maximum pressure. Maximum pressure at maximum operating temperature would be the worst combination."

I don't agree that the heated tire is more likely to burst under pressure than a cool one. Tires are designed to operate at certain temperatures, which for the most part is better when hot than cold. Race tires are an example of that.

.

[woodturner]

I have always been very careful in following the car sticker, and rarely allowed it to go above 32psi; 40 seems to be vastly over-inflated

The number on the sidewall of the tire is a maximum pressure rating, not a recommendation. It's not really safe to run tires at that pressure - all the manufacturer is saying with that number is "never inflate the tire above this pressure". As others have noted, pressure increases with temperature, so the practical maximum is around 6 psi less than the sidewall number.

The tire manufacturers make tires to meet industry specs, and they can be used on many different cars. Only the vehicle manufacture knows the vehicle design and loading and has the information to do the engineering analysis. As others have noted, they are also the ones with the liability. (BTW, at least some vehicle manufacturers do warrant the tires).

Bottom line, only the vehicle manufacturer has the knowledge and information to be "right", and tires should be inflated to the vehicle manufacturer's recommendation.

If you do find tires are wearing abnormally, it often means the tire pressures are not being checked weekly and are not really at the recommended pressure, or are not being rotated every 5,000 miles. However, if the tire pressure is being maintained and the wear is still abnormal, it's OK to adjust the pressures 1 to 2 psi to even out wear.

[cabinetman]

If you do find tires are wearing abnormally, it often means the tire pressures are not being checked weekly and are not really at the recommended pressure, or are not being rotated every 5,000 miles. However, if the tire pressure is being maintained and the wear is still abnormal, it's OK to adjust the pressures 1 to 2 psi to even out wear.

If the tire pressures have been checked regularly and the wear is abnormal, the likelihood of a mechanical/suspension/worn/damaged, part could be the problem, and adjusting the pressures 1 or 2 psi would not be the fix. That's false hope, and could lead to the possibility of an unsafe condition. If abnormal wear exists, it's best to take the car to a certified mechanic for evaluation.

.

[TB Roye]

The tire pressure marked on the tire is the Max pressure when the vehicle is fully loaded. Back in the 90's when Ford and Firestone were having problems with the the Explorers and tires blowing out and the vehicle crashing. The issure was tire pressure People were complaining that the SUV rode like a truck DUH! so Ford lowered the tire pressure for ride quality. So people were running around with 26PSI in their fully loaded SUV at 70+ mph and the tire failed. Granted there were problem with the Firstone tires also, but the low tire pressure was the main problem. The under inflated tires were over heating and blowing out. Over inflation will cause the tire to wear in the center and low pressue on the edges. Alignment, Suspension wear, Balance will cause a whole different wear pattern.

Tom

[radhak]

I think all this while I might have over-spent a lot money on tires by not having this discussion earlier !

I am going to give equal credence to both the car and the tire manufacturer and keep my psi to 36/38 : right in the middle of their recommendations.

I better take a look at wife's car too - methinks she's been changing tires a bit too fast (of course, in her case it must be because she'd rather take it to the mechanic for 'an opinion' instead of asking me; nobody believes a spouse !).

[Cochese]

I'll say this:

I've worked on cars for as long as I could drive them. I always used to go strictly by what the recommendation on the door said. While I no longer do that, it's a good starting point.

Why?

Car manufacturers don't care about tires. They aren't covered on any warranty I'm aware of. If a tire fails, usually it doesn't affect the vehicle itself or how someone views the car or car manufacturer, except in the case of the Exploder/Firestone case. Car manufacturers care enough about the tires for the consumer to expect two things: comfort, and the rated gas mileage.

[cabinetman]

I think all this while I might have over-spent a lot money on tires by not having this discussion earlier !

I am going to give equal credence to both the car and the tire manufacturer and keep my psi to 36/38 : right in the middle of their recommendations.

I better take a look at wife's car too - methinks she's been changing tires a bit too fast (of course, in her case it must be because she'd rather take it to the mechanic for 'an opinion' instead of asking me; nobody believes a spouse !).

Some cars have recommendations for different pressures for the front and rear. That should also be followed.

.

[woodturner]

Car manufacturers don't care about tires. They aren't covered on any warranty I'm aware of.

After the 2001 Firestone tire fiasco, Ford changed their warranty to cover tires
http://www.autonet.ca/autos/news/200...2/4818089.html

The manufacturer also cares because they are the ones with the liability and "ownership" of the overall vehicle design and operation.

I assume other manufacturers warrant the tires too, but I have not checked that so I can't say with certainty.

[Cochese]

After the 2001 Firestone tire fiasco, Ford changed their warranty to cover tires
http://www.autonet.ca/autos/news/200...2/4818089.html

The manufacturer also cares because they are the ones with the liability and "ownership" of the overall vehicle design and operation.

I assume other manufacturers warrant the tires too, but I have not checked that so I can't say with certainty.

Here's current warranty information: (Fusion, in this example)

http://www.ford.com/cars/fusion/2011/warranty/

For vehicles within the New Vehicle Limited Warranty time in service
and mileage coverage period, defective tires will be replaced on a pro
rata adjustment basis according to the following mileage-based
Reimbursement Schedule:
MILES DRIVEN PERCENT OF PARTS COVERED BY FORD
1-12,000 100%
12,001-24,000 60%
24,001-36,000 30%
The tire manufacturer also provides you with a separate tire warranty that
may extend beyond the New Vehicle Limited Warranty coverage. You will
find the manufacturer’s tire warranty with the owner literature supplied
with your vehicle. You have the option of having a tire warranty repair
performed by the tire manufacturer’s authorized service center. If you go
to a tire service center for a repair covered by the New Vehicle Limited
Warranty, you may be charged a prorated amount for wear or other
charges




The New Vehicle Limited Warranty does not cover normal wear or worn
out tires. Tires will not be replaced (unless required by a warranty
repair) for wear or damage including:
• tire damage from road hazard such as cuts, snags, bruises, bulges,
puncture, and impact breaks
• tire damage due to under or over inflation, tire chain use, racing,
spinning (as when stuck in snow or mud), improper mounting or
dismounting, or tire repair

[dbhost]

Sorry I didn't respond last night, too tired... But let me give you my perspective.

I haven't mentioned this much here because it was never relative to our discussion, but Information Technology wasn't my first career. My first degree / career was an Associate of Applied Science in Auto Diesel Technology, I was an ASE Certified and degreed auto mechanic. I specialized in suspension systems, braking and air conditioning (quick, easy money). I did a LOT of repair work as well as custom builds, lifts, lowerings, etc... I changed careers after moving to Texas. (Too hot / humid to work in these shops here!)

One of my previous employers WAS Firestone, and I can tell you without any hesitation what so ever, that the problem with the Ford Explorers was two fold, poor quality tires, and badly under inflated tires.

The typical 35PSI MAX rating on P rating type tires (Passenger car tires) is just what it says, MAX rating. This should be set with the tires COLD. This is set to allow for the maximum sidewall stiffness and tread contact with the tires rated load on it.

My F150 came with load range C tires (35PSI max) and the door jamb sticker says 32PSI front / 35PSI rear. I believe that with THOSE tires.

I presently have (at least until I get some new shocks / tires) a set of load range D mud tires on that same truck, 45 PSI max. Running those tires at 35PSI on the street is not a good idea. too much sidewall flex, causing unstable handling. I keep them inflated to 40 PSI with no load on the truck and they work great. That is also the recommendation of the tire manufacturer.

Vehicle factory specifications are good ONLY if the vehicle, or at least the subsystem of the vehicle you are dealing with meets factory specs. When you deviate with different tires, shocks, bushings etc... the whole dynamic of the car changes. Tire pressures change, but as a general rule of thumb, I do not keep tires at anything less than 5PSI lower than the rated max of the tire..

[woodturner]

Around 2001, the Firestone Wilderness AT tire was in the news. Ford said the tires were defective, Firestone said they were underinflated. Ford argued that the Goodyear tires worked fine at that pressure, and pointed out that other manufacturer's were having excessive failures with the Firestone tires, even when inflated to Firestone's recommendation. Around 250 deaths were reportedly caused by those defectived tires, with 119 ocurring on Ford vehicles and the others on other vehicle brands.

[dbhost]

Moderators note:

Folks, I know on certain topics, and with certain historical events passions can run deep.

However that does not change the rules.

Please refrain from inflamatory language and personal / character attacks on both individuals and institutions such as businesses, governments, schools and non profit organizations.

It is our sincere effort here to keep the tone of BT3Central a family friendly, reasonably anger free place where we can all enjoy the time we spend here.

[JimD]

For normal use, my practice is to inflate the tires on my vehicles (two at the moment, a SUV and a convertible sporty car) to 2psi more than is shown on the door jamb. The SUV is on it's second set of tires, the convertible still has the OEM run flats. I autocross the convertible occasionally and I set the tire pressure for that purpose based upon testing. There are little triangles molded into the tire at the junction of the tread and sidewall. I put the white marking stuff you use to write a price on a windshield on the side of the tire and made a run. I went up in pressure until the white stayed on the sidewall and only wore off on the tread. That was at 40psi. I take it back down before driving home (34 on the fronts, 38 on the rear since the door jamb says 32/36).

I over-inflate a little because it helps mileage, it helps make up for the fact I don't check it as often as I should and it goes under some too, and it still rides OK.

If I could find a tire manufacturers recommendation for their tire on my car I would use that. I think using the car manufacturers recommendation for their car with OEM tires is better than using the maximum tire pressure the tire can take. When I change the vert to regular tires, I will start at my normal practice and see how they wear. If I notice abnormal wear, I may shift things a little.

Jim