Max Tire Pressure?

Awl--
I noticed that the recommended tire pressure for a vehicle is usually well below the max. printed on the tire. For example, on my new Honda Fit, the door says 32 psi, but iirc, the tire
says 40 or 45 psi.
What goes into this recommendation by the car mfr? (Recall the Firestone tire debacle, which I heard was largely caused by Ford recommending unrealistically low pressures, for a cushier ride.)
It seems to me that filling the tire to the *tire mfr's* printed max should be OK, giving me 1. better mpg's 2. perhaps a bumpier ride, and 3. perhaps less traction/longer braking distance than the car mfr's recommended psi.
Since I drive pretty much slow as shit anyway (or with the "flow of traffic"), mpg's are my main priority, as long as I'm not positively risking my life. I've noticed no uneven wear on previous cars, by filling to the max.
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Mr. P.V.'d (formerly Droll Troll), Yonkers, NY
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wrote in message

Wrong on two points. The recommended pressures were not "unrealistically low" and they were not recommended "for a cushier ride." When testifying before Congress, even the Firestone Executive who was trying to shift the blame to Ford, admitted that the recommended pressures were well within the industry standards for tires of that size and type. The recommended pressure was similar to the pressure recommended by Toyota and Nissan for SUVs with similar size tires and loadings. Around half of 1996 Explorers were sold with Goodyear tires instead of Firestone tires. The Goodyear tires got exactly the same pressure recommendation and did not experience abnormal failure rates. Ford also installed the same bad Firestone tires on Ranger pick-ups, but with a higher pressure recommendation for the rear tires, and they still failed at an excessive rate (my Father had three of four fail on his Ranger). Ford picked the tire pressures to reduce steering response and increase understeer. This was to prevent people from breaking the rear end loose and possibly spinning the vehicle or turning it over. Firestone was well aware of Fords recommendation and made no objections until after the tires started failing at an excessive rate. Firestone built defective tires and tried to blame Ford. Ford isn't clean since they were stupid enough to buy second rate tires from Firestone.

1) While increase tire pressure may reduce rolling resistance and improve gas mileage, it is not a straight line effect. Past some point, increasing the pressure further will not significantly affect fuel economy. Given the pressure of CAFE regulations in the US in is very unlikely that recommended tire pressures are so low as to greatly reduce fuel economy.
2) Higher pressure will definitely result in a stiffer ride. You may prefer this. On the other hand, higher pressures also make the tires more vulnerable to damage from stones and broken pavement. Higher tire pressure also result in a higher level of road shocks being transmitted to the suspension on body.
3) Handling can be greatly affected by tire pressure - particularly the front to rear differential. Excessively high pressure can definitely increase braking distances and reduce cornering ability. High pressures can also make the steering very "nervous."

Radial tires are much more forgiving of incorrect pressure than bias ply tires. I own a Nissan Frontier, and even with the recommended pressure in the rear tires I am experiencing excessive wear in the middle of the tread. Nissan only recommend "one" pressure for the tires. I am sure this is a "safe" pressure and one that results in maximum gas mileage. However, for a lightly loaded truck, the pressure is likely too high. Years ago, manufacturer's usually recommended different pressures for different conditions (speed, load). In recent years, this has been replaced by a one pressure fits all situations recommendation. It is my belief that this pressure is probably higher than necessary for most conditions since they have to allow for maximum load and speed and the manufacturer's need to achieve the highest possible mileage ratings for CAFE.
From http://www.michelinman.com/care/tip1.html :
"Recommended Pressure
"Always inflate your tires to the recommended pressure listed by your vehicle's manufacturer. This information can be found in the owner's manual and often on a placard located in the vehicle's door jamb, inside the fuel hatch, or on the glove compartment door."
From http://www.goodyeartires.com/faqs/Inflation.html :
"How much air should I put in my tires?
"Proper inflation is the single most important part of tire care. The inflation pressure on the side of the tire is the MAXIMUM operating pressure. It is not necessarily the right inflation for your vehicle. Always use the inflation recommended by the vehicle manufacturer. You can find it in your owner's manual, posted on the edge of the driver's door, on a door post or on the inside of the glovebox door. Always check inflation when tires are COLD: when the vehicle has been driven less than a mile or one hour or more after driving. Use a good quality tire gauge. Note: It's natural for radial tires to have a slight bulge in the sidewall at their proper inflation pressure. Check or adjust inflation every few weeks, before any long trip or if traveling with a heavy load. And don't forget to check the spare. Your Goodyear retailer can answer any questions you may have about tire inflation."
From http://www.tiresafety.com/maint/maint_ipressure.asp (this is a site linked from Firestone's home page):
"Inflation Pressure
"Proper inflation pressure is essential for achieving maximum performance and mileage. Improper tire inflation pressure can cause severe internal tire damage, which can lead to sudden tire failure and resulting in serious personal injury or death.Improper inflation pressure may result in rapid or irregular wear. Pressures should always be checked when the tires are cold and at least monthly. Under normal tire operation, approximately 1psi of tire pressure will escape every month. Also, for every 10 degrees F change in ambient temperature, tire pressure will change by approximately 1psi.
"Vehicle manufacturers list recommended tire pressures for original vehicle tires in the owner's manual or on a placard on the end of the driver's side door or in the glove box.
"For continuous high speed driving, tire pressures should be increased by 3 to 5psi above the normal cold inflation recommended. However, for passenger tires, never exceed the maximum inflation pressure molded on the sidewall. The inflation pressure for light truck tires may exceed that molded on the tire by 10psi. Any recommended front to rear pressure differential should be maintained."
From http://www.conti-online.com/generator/www/us/en/continental/automobil ...
"Tires are designed and built to provide many miles of excellent service but must be maintained properly. The key element of proper tire maintenance is maintaining the recommended tire inflation pressure. The proper tire inflation pressure is recommended by the vehicle manufacturer and can be found on the vehicle's tire placard or in the vehicle owner's manual.
"Continental Tire recommends that the consumer check his/her tire inflation pressure at regular intervals of at least once per month and before every long trip or twice per month depending on local regulations, customs, or conditions."
From http://www.coopertires.com/Flash/index.aspx :
"It's important to have the proper air pressure in your tires, as underinflation can lead to tire failure. The "right amount" of air for your tires is specified by the vehicle manufacturer and is shown on the vehicle door edge, door post, glove box door, or fuel door. It is also listed in the owner's manual."
Ed
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C. E. White wrote:

<snip>
fwiw, my 2001 Trans Am's sticker has a 30psi rating for normal driving, and a 38psi rating for sustained speeds above 100mph. I love that sticker. :)
Ray
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Wow, I am surprised GM doesn't get sued over that one. I mean if the sticker says you can go 100, you probably should....
Do you remember when manufacturers were installing tires with an 85 mph maximum speed rating? I thought that was insane, even though the National Speed Limit was 55. Now things have gone in the opposite direction - cars come with tires rated for 135 mph, even if they have governors limiting them to 105. And tire stores force you to buy the higher rated tires even if you never plan to go over 80....
Ed
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Nobody is selling cars with tires rated for only 75 mph today in the US. I can't say what is happening in the rest of the world. My Ford Fusion came with P225/50VR17 - 93V tires - they are rated for 149 mph. If I go to Sam's or Costco to buy replacement tires they will insist that I buy tires of the same speed rating. There is no way that Fusion is going to do 149 mph unless I drop it off a clift.
Ed
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C. E. White wrote:

The actual sticker says:
"See owner's manual for tire pressure needed at 160 KM/H (100MPH) or higher speeds, where legal, and for additional tire information."
The owner's manual (which I don't have handy) lists the 38 psi as the pressure required for driving above 100.
Considering in stock form, this car can push 160mph, I see why the sticker exists. I don't know what percentage of F-Body (Camaro/Firebird) owners race their cars, but it's pretty high.
Ray
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Tires are fairly "generic" and are designed so that they can be used on a host of different vehicles of different weights. The manufacturer sets a recommended pressure for the specific vehicle taking in to account things like vehicle weight and handling characteristics.
If you run the tires up to the max limit you end up riding on the middle of the tire. Now the tires aren't going to last as long as they should and your handling / braking goes to pot. Any potential savings are quickly negated by having to replace the tires early.
Vehicle manufacturers have nothing to gain by intentionally making their vehicles get worse gas mileage. If you want to save gas I would work on keeping the tires properly inflated, make sure the engine is in good tune and most importantly work on driving habits that might be costing you gas mileage.
Steve B.
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Proctologically Violated wrote:

I like that the recommended pressure isn't higher than the tires' capacity. But that's just me.
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Proctologically Violated wrote:

We run mostly oversized tires on our off road Jeeps so we need a way to find the compromise for best traction and best handling vs gas mileage and safety.
We use the chalk method of running a few lines across the tread and driving straight, then stopping to see how much is left. The best for grab is usually no chalk except about 3/8" at each edge. This is lower than the mfg vehicle spec which gives about 1" on each edge.
When my 33x9.5" tires to use one specific case are at 26 psi I have all 7.5" of tread touching but a soft sidewall. Good for slow speed only.
At the vehicle's spec of about 30 psi, I have about 3/4" of chalk on each side. At the max of 45 psi for the tire maker, I only have 3 1/2"- 4" of tread touching the ground! This leaves a gap that just screams for a little water to get lift off!
I run at 28-29 psi and get good wear with decent handling and decent enough mileage which is a compromise. If loaded I run at 32 psi though.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id !15147590 (More Off Road album links at bottom of the view page)
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Neat trick, the chalk thing. Elegant, simple, revealing.
Good thread. I'm deflating my tires as I type. :)
Altho I did read someplace that underinflation leads to more punctures than high-er inflation.
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Mr. P.V.'d (formerly Droll Troll), Yonkers, NY
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Proctologically Violated wrote:

If you ask 10 people their recommended tire pressures, you'll probably get 11 answers.
When in doubt, with stock tires on a stock car, stock pressures are usually good enough.
Once you start changing sizes, widths, compounds, tire types (radial vs bias ply, street tire vs race tire), racing, offroading, lifting, lowering, trailering... then you need to worry. :)
Ray
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