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Tire Pressure?

Posted by pawprint1 (My Page) on
Sun, Nov 6, 05 at 10:21

Am I the only one who thinks the factory recommended settings are too low?

What are yours set at? Do you keep the back tires a few pounds lower?


Follow-Up Postings:

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RE: Tire Pressure?

I guess it goes back to the "if a little's good, a lot's better" way of thinking, but I usually do put my tire pressures a little higher than what the manufacturers' recommendations state. People I know who go to quick oil change shops say those shops usually put 32 - 35 PSI in their tires, regardless of the factory recommended pressures.

One thing I always think is that the recommended pressures probably have most significance for the OEM tires. If I replace the original tires with a different brand or model, I usually split the difference between the maximum pressure stamped on the tire sidewall and the manufacturer's recommended pressure.


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RE: Tire Pressure?

The manufacturer places the PSI figure in the owner's manual (generally unread) and in rather small print on one of the many labels on the hood, engine bay, and door areas. Then there are figures on the tires which have little to do with normal running pressures...
People are foolish indeed to expect quality auto service at the "quickie oil change emporiums"..


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RE: Tire Pressure?

I would follow the manufacturers recommendation. Why would they mislead you? Or is it that you got smarter than their engineers? The figures on the tires are an average figure and do not take into consideration the end weight of the vehicle. But heck, you know don't you?


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RE: Tire Pressure?

Don, This misleading of the customer is exactly what Ford and Firestone did about ten years ago with the Explorer tire pressure. Ford tried a too low pressure to try to make a truck ride as a car...Rollovers were the result..

And one I know of a dealership in Bayport, Long Island that recommended 18 PSI when the factory stated at least 25, maybe 28, this was over 30 years ago..
The tire figures are for the PSI for the maximum load('97 Accord 560 kg per tire)....
A man must be careful...


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RE: Tire Pressure?

Maybe - Watch the tread wear patterns.
Too much and they wear out quicker in the center.
Too little and they wear out quickly toward the outside edges.


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RE: Tire Pressure?

I was under the impression that treadwear patterns only worked with older bias ply tires.
if the radial tires are wearing unevenly you need an alignment.
John


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RE: Tire Pressure?

Manufacturers set their tire pressure recommendations for a variety of reasons, and one of them is to provide a soft ride. It's common knowledge that adding pressure makes the car ride harder, and it also improves gas mileage. (More air in the tire equals less rolling resistance.) This isn't a case of someone thinking they know more than the engineers; it's a case of someone making an informed decision to trade some ride quality for fuel economy, longer tire life, and in some cases better handling.

The number printed on the tire is a maximum pressure rating for the tire, not an average rating, and it does take into consideration the load rating of the tire. The heavier load you are carrying, the more air you put in the tire to keep the tire from getting too hot (but not exceeding the tire's maximum pressure rating, of course).

Lots of things can make radial tires wear unevenly. Worn shocks or struts, for example, can make the tire cup. A badly cupped tire will cause the car to make a "clop, clop, clop" sound as it goes down the road, and it will also vibrate in a similar way to what a tire does if it's out of balance. Underinflation or overinflation will also still affect the wear of a radial tire, but not as much as a bias ply because the radial's design helps it maintain its shape better even if over or under inflated.

My understanding about the Ford Explorer/Firestone tire problems is that it was a case of people not properly checking their tire pressures. If these people had followed either the tire's maximum pressure rating OR Ford's rating on the placard inside the door, they would have been fine. But when you run a tire that's under-inflated, it overheats. In the case of these Firestone tires, they were not sufficiently over-engineered to be able to tolerate that, and the tires failed. People do not make tires fail by putting the pressure a few pounds above the car manufacturer's recommendation but within the tire manufacturer's maximum. They make their tires less likely to fail, in fact.


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RE: Tire Pressure?

That is the main reason I asked. I put in more than my car recommends. No, Im not an engineer, but I get better gas mileage out of more air.

I also put more in than the recommended pressure for winter because I read that leaving my heated garage into the cold mornings (soon to come) makes tires loose pressure.

Lastly, by the time I check the pressure again, its back to the manufacturer recommendation anyway. (I know, lame excuse, but true)


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RE: Tire Pressure?

Often, the pressure rating on the sidewall of a tire is its maximum pressure rating, not the recommended pressure for useage. The max rated pressure should never be exceeded even when mounting a tire. When this pressure is exceeded, internal damage to the tire might result, but not be readily apparent, for example belt stretch or breakage. This belt damage may show up later, say in 8000 miles when the tire begins to "thump" at speed, and can't be balanced because its changing shape at speed.

The "useage" pressure will vary depending on many factors such as the tire size and load rating vs pressure, vehicle weight, load balance between front and rear wheels, braking behavor, handling, road wander, reaction to side winds, ride quality, etc. In my experience, most passenger cars will have a pressure range between 25 to 35 psi. Large minivans and truck based vehicles with stiffer truck tires may have higher recommended pressures. I find, that on the average (for passenger vehicles), I get the best handling with the front tires running 2 psi greater than the rears. However, this is not a given since this can change depending on weight distrubution and the side stiffness of the suspension system.

So, back to the question, which pressure recommendation to follow? In my opinion, the vehicle manufacturer's recommedation is the best starting point unless it has been proven in error. There is merit to the tire manufacturer's recommendation in the case of load rating vs pressure. I would be leery of recommendations on the low side, or below, that of the pressure range recommended by the tire maker.


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RE: Tire Pressure?

Right, what's stamped on the sidewall is a maximum pressure, not a recommended or average pressure. In the first post in response to the original question, I said that if I replace the OEM tires with a different brand or model, I usually split the difference between the car manufacturer's recommended pressure and the maximum tire pressure stamped on the tire's sidewall.

One thing to keep in mind here is that adjusting tire pressures is, by necessity, a somwhat inexact science. If you go buy a precise gauge and set the tire pressures at 32 psi after they've cooled off tonight, then take off on a long trip at 6 o'clock tomorrow morning when the temperature is 15 degrees cooler, the tires will have a couple pounds less pressure in them than what you put there. Then after driving for three hours on the Interstate, they'll have more pressure in them than what you put in there.

For people who live in areas with changeable climates, it's entirely possible to have a 60 or 70 degree difference in temperature between the lowest and highest temperature encountered in just a few days. This alone makes a big difference in what the tire pressure readings are. Then add to that the heat from driving, which makes the pressure vary still more.

The bottom line is, make sure your tire pressure is no lower than what the car maker calls for and no higher than the maximum stamped on the tire sidewall.


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RE: Tire Pressure?

Time to change over to winter air ...right about now ?


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RE: Tire Pressure?

Then there is the VW van, from the 70s, with radial tires.
Fronts 25 psi
Rears. 43 psi.... and they still appeared to be under inflated !
And how about, for the 300 pound driver, without a passenger, placing 2 extra psi one the left front tire ???


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RE: Tire Pressure?

I inflate mine as instructed on the door sticker or owners manual of the vehicle. When I have tires replaced, I note if what the manufacturer of the vehicle says is consistant with what the manufacturer of the tire says. Always has been in my case. I also note how the tires are wearing. If they're wearing faster in the center, I run a little less air pressure. If they're wearing on the shoulders, I add a little more. Either way, I stick within the recommended range.

As to the Ford Explorer/Firestone thing, yes Ford recommended a little lower air pressure when the vehicle was not loaded. The tire still shouldn't have flown appart like they did. Remember Firestone 500 tires? I just think they cheep out sometimes trying to save a buck. Got spanked at least twice for it now.

A lot of the roll over accidents due to tire failures were partly due to fools driving the things 80+ mph on a 100 degree day. A Ford explorer isn't a ferrari. People get into trouble when they try to drive it, or most any other 4x4 like it is one. jmo


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RE: Tire Pressure?

I would look to the tire itself for the PSI rating on the sidewall, rather than the vehicle manufacturer.


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RE: Tire Pressure?

Ok, so the manufacturer picks a tire pressure and this is pasted on every car made. BUT, even the same brand and make can have different tire manufacturers. SO, does the air pressure change for brand of tire? Or make of car? And what if they are figuring my car on holding 4 people, when I rarely if ever even have 1 passenger?

I still think the manufacturer setting is too low. 19 in the rear. The tires even look flat at that pressure.


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RE: Tire Pressure?

Yes, the car maker posts a recommended pressure for the original equipment tires. As you said, some cars have more than one brand of tires as original equipment, so the factory settings would be valid for any of the models of tires they use as original equipment. (Any factory original equipment such as tires has to meet certain specs, so the tires from different brands are probably of similar design and quality.)

A recommended pressure of 19 psi in the rear is unusually low. I know that a few cars throughout history, such as the Chevy Corvair, had odd tire pressure recommendations, which in the case of the Corvair I believe were quite low. GM said those pressures were necessary to correct the handling oddities of that car which were detailed in the book "Unsafe at Any Speed," by Ralph Nader. Then people realized that the VW Beetle had a set of handling irregularities similar to those of the Corvair, so another book was released to detail the Beetle's alleged safety problems, and VW then called for some unusual tire pressures as well. Of course, both cars' handling "problems" were inherent to their rear-engine design. A Porsche 911 can be very tricky to drive, too, if a person isn't used to the type of weight balance you get on that design of a car, so I wouldn't be surprised if that car has unusual tire pressure specs, too.

But few cars today exhibit these types of odd handling, so the 19 psi recommendation you have is probably put there primarily to improve ride quality. But I don't know that; you might want to check with a dealer. I would not be surprised if many people at the dealership aren't even aware of that recommendation, and when they service these cars they just put 30 or 32 psi in the rear tires. You may also want to check with a tire dealer selling your brand of tires to discuss this 19 psi recommendation and see if they have any comments on it.


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RE: Tire Pressure?

pawprint, what kind of car is this? What tire? 19 seems low. Please confirm the spec you're going by. If it says 19, make sure they're refering to PSI. Not everywhere uses PSI as the unit of measure.

On corvair's and VW's, they recommendend higher air pressure on the rear to support the weight. Porsche deals with it by putting bigger tires on the rear. Both of those ways of dealing with weight are common on trucks.


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RE: Tire Pressure?

The low air pressure recommendations for the Corvair were for the front:

"In addition, the car was designed to avoid terminal oversteer by using very low air pressure in the front tires, typically twelve to fifteen pounds per sqaure inch, so that they would begin to understeer (slip) before the swing axle oversteer would come into play. Although this pressure was quite adequate for the very lightweight Corvair front end, owners and mechanics, either through ignorance of the absolute necessity for this pressure differential between front and rear or thinking that the pressure was too low for the front, would frequently inflate the front tires to more "normal" pressures, thus ensuring that the rear of the car would lose traction before the front, causing it to spin."

Here is a link that might be useful: Answers.com Chevy Corvair page


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RE: Tire Pressure?

A very interesting story about the Chevrolet Corvair from Answer.com.
A few errors of course.
There was no "Nash Rambler" to compete against the Corvair.
Both Nash and Hudson were dropped as names in either '57 or '58.
The all-new" Rambler, introduced in '56, continued on for about 10 years, successfully keeping the new American Motors alive...
The Corvair was a flop, GM realized this in '61 and introduced the Chevy II to do battle against the Ford Falcon...and the Plymouth Valiant, Rambler and Studebaker Lark.

One must give credit to GM for having no fear to innovate - but the Corvair design(and the VW air-cooled as well) were simply second place...to a Corolla or a Falcon..
The VW tire pressures were 17 to 18 in the front and 25 to 28 in the rear( as I remember)..
Sadly, for many people, this was too complicated...
All I remember of the Corvair are pressures of 25 in the front and 35 in the rear - I never owned nor serviced one..


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RE: Tire Pressure?

**The Corvair was a flop,**

I thought it was very popular. I was under the impression it was Nader singling it out to point out safety problems, metal dash board, no seat belt, no roll over protection, no crumple zones, ect, that killed it. I remember him saying he could have used any car made in those days to make the point. He just picked the corvair as an example, and that scared people away from it.


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RE: Tire Pressure?

IMO, what Nader did was wrong!
The real problem ,IMO, is that people were too lazy to read the manual and maintain the correct tire pressures. the "conventional" cars were better able to handle improper or lack of maintenance..
But I think the Valiant and Falcon were better cars - as reflected by the quantity of sales...
I am sure that a '65 Corvair, with correct pressures was an excellent handling car...driven by a professional, and not by some maniac...


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RE: Tire Pressure?

Mom had a 60 covair and replaced it with a 65. I can remember borrowing her 65 for some reason and approacing a traffic signal as it changed one rainy night. Applied brake and instantly did a 180 spin. Found myself driving backwards in the same lane at about 45 mph.

That traffic signal was screwbally, yellow on one side and red on the other.


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RE: Tire Pressure?

FWIW, when I went to Skip Barber's advanced driving school at Lime Rock, the instructors unanimously suggested filling tires well over the sticker in the door jamb. One even said all the way up to max sidewall pressure.

Now I know that group of folks come from a very unique perspective but I nevertheless fill mine about 3-5# higher than the sticker. Can't bring myself to go much over 35#..


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RE: Tire Pressure?

I think that's a good point, and most "performance oriented" drivers probably would agree with it. I mentioned the Corvair to make the point that there are those times when it's pretty important to follow the car maker's recommendations to the letter, but that's the exception.


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RE: Tire Pressure?

A little extra air beyond the specs is all right, but if you get it too high the tires will wear unevenly. Conversly, too little air pressure will also cause uneven tread wear. Too much, and the middle wears out first; too little and the outside tread wears first.


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RE: Tire Pressure?

I run 32 psi. I usually put in 35 psi right before a trip. The reason is tire life and fuel economy. Seems to me that no matter how I inflate tires, the front ones always wear the edges out and the rear ones don't. So I rotate them front to rear at half life (30,000 miles). Whenever I've checked my tire pressure right after getting new tires, it seems they've always been inflated to 32 psi by the place that puts on the tires. I have heard that for the longest tire life, run your tires at the max pressure printed on the sidewall.


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RE: Tire Pressure?

The fronts are going to wear on the shoulders due to camber and toe-in adjustments. On solid rear axle cars, the tires wear flat when properly inflated.

How well they wear to how much air is in them depends on how much they're loaded. If there's not much weight on the tire and they're filled to capacity, they'll wear in the center. Not enough air, they wear on the shoulders.

You'll probably get a smidgen better gas mileage with the tires filled to capacity, but they'll wear in the center sooner. So, get 1/2 mpg better hiway and replace tires at 40k, or pay attention to tire wear, adjust pressure accordingly and get 50k +, perhaps at the cost of a little less fuel economy. Either way you're going to be paying somebody for something. jmo


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RE: Tire Pressure?

Posted by: pawprint1 (My Page) on Wed, Nov 9, 05 at 19:56

That is the main reason I asked. I put in more than my car recommends. No, Im not an engineer, but I get better gas mileage out of more air.

I have been privileged to know many people o'er the years. Many of those " on the street" are simply more intelligent than the average engineer - some engineers are downright stupid about some things, but then credit must be given to them for their career status...

And I still hold that it is best to go by the PSI figures in the owners manual - for my Saab 900 that could be 28 to 39 PSI, the factors being the load, the tire, and the speed. The figure on the particular tire itself is set by the TIRE manufacturer, in my case this happens to be 35 PSI - which is a MAX figure, and, IMO running at several PSI over this when driving on snow and ice will be beneficial, and hurt nothing .....


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RE: Tire Pressure?

We had a tire guy here, he may still be out there... did a big business with race car tires of all sorts. Anything, he could get and/or recommend.
He had a tire truing machine that would peel a ton of rubber off tires to make them perfectly round on the rim.

There was almost always traveling sales type guys with the caddy's and mercedes standing around this grubby little tire shop swearing how smooth their rides were after his align/balance and turned true tire cuts. People came from far away for his services.

He was a proponent of the fill it to the sidewall label pressures for street driven tires. If its too ruff, come back for some of these shiny shocks. He knew more about the tires it seemed, than the factory guys.


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