Posted: 4/17/2008 10:31:49 AM EDT
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Ok, your vehicle has a recommended tire pressure, and your tires have a maximum tire pressure. For academic purposes, let's assume a passenger car, or light truck, not a sports car in an autocross situation. I'm assuming that the maximum tire pressure (on the tire) is the max pressure at which the tire maintains its proper shape/contours under load. If this is the case, for economy purposes, why wouldn't one simply go to the maximum tire pressure as listed on the tire. The ride might get a little harder, but the rolling resistance should drop big time. Sure, there will be a handling difference in cornering, but stopping on pavement should not be hampered that much. I'm assuming the tire pressure listed on door sill is specific to the OEM tire, and is a parameter for maximum handling. This obvously has legal liability issues, so no manufacturer would ever say 'take it to the max on the tire', because someone would sue because of the marginal % decrease in performance. Am I wrong? Would I be foolish to take my tires to the maximum pressure listed for ordinary commuting? |
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IIRC my door sticker says 32psi and the max pressure listed on the tire is 35psi. I consider it splitting hairs because I'm sure my crappy tire gauge is off by that much. I generally run at the max listed on the tire unless it differs significantly from the mgr recommended. -JTP |
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Usually the manufacturers recommended pressure is for the best ride. The tire sidewall flexes more resulting in a smoother ride. The maximum tire pressure on the sidewall of the tire is for a fully loaded vehicle driving at highway speeds. I usually run about 10% less for in town driving. On customers cars I check pressures when they are on the lift and if they are too low and they usally are. I will fill the tire to maximum pressure so it will have enough air in the tire to make it to the next oil change. I also look for nails and screws and repair as necessary. Check your tire pressure with a guage every 2 weeks to make sure you don't have a slow leak or puncture. One thing I have seen is to mark the tire across the tread with a tire crayon every 30 degrees or so then drive it for a bit and see how the crayon is worn off. If it is worn off on both edges the tire pressure is too low. If it is worn off only in the middle then the pressure is too high. This is a rough test of tread contact and you may need to fudge it up or down. |
| Max air pressure would ride like crap too, with no load. Everytime I go to get my oil changed they put 35psi in my tires and the ride beats the crap out of me until I get home. The next day, once the tires have cooled I let air out back to where I normally run it. My tires and wheels are not stock size, so I have found, through trial and error, the best air pressure for me to use. |
My Dad had a Buick Skylark (1995) that rode terrible with 32 psi but was great with 28psi it even had the Dyna-Ride package. |
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The sidewall "max pressure" is given for a specific load. If your corner weight is different then those numbers don't mean much. You're going to have to drive the car and experiment with tire pressure to find out how the car responds to different pressure. Someone in another forum suggested rolling the car through standing water and observing the contact pattern to see if you're over inflated. I suspect there may be some use to this method. |
You rock.  If I may add: Running the tires at the maximum pressure listed on the tire's sidewall will not increase wear in the center of the tire, or anywhere else for that matter. If fact, it will probably reduce the tire wear. Tire wear comes from heat. Heat comes from under-inflation. Blow-outs are caused by too little air, not too much. On some vehicles, there is no "range" between the door sticker and the tire's sidewall. The rear of 3/4 and 1-ton single rear wheel pickups is a good example. They will typically specify a load range "E" (10-ply rated) tire, inflated to 80 psi, and the tire's sidewall will list the maximum pressure as 80 psi. Problem is, a lot of goof-balls buy these trucks to use as commuter vehicles, and they don't like the fact that it rides like a truck, which is ironic, because it is a truck. So they run the tires low. Sometimes with less than half the air that they should have. So it rides soft and they like it, until it causes a blow-out (usually loaded or towing a trailer), or they ram something when the ABS computer gets confused and shuts off the brakes because the rear tires have no traction anymore. Extreme goof-balls actually down-grade the tires to a load range "D" or even a "C" (like on a 1/2 ton truck), because they like the "look" of larger tires, but they can't find, or don't want to pay, for over-sized load range "E" tires. The larger loaded radius also can burn up brakes and transmissions if they actually use it loaded or with a trailer. |
Quoted by KirkP
Do you think the Firestone tire fiasco was caused by underinflation? I was told that Ford was recommending as low as 24 psi to improve handliing. In my experience lower pressure in tires will make it handle like the tires are flat. 
Very true! I recommend the origional size tires that the vehicle was designed to have. Putting taller tires on which you get with a wider tire due to the aspect ratio will reduce your steering lock. Among other things. 
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Most tires will meet their maximum load rating capability at a pressure well shy of the maximum pressure rating. That is the main difference between a P235/75R15XL and a LT235/75R15 LRC. The load capability of the tires are roughly the same, but the max load with the XL comes at a lower pressure than the LRC while the max pressure remains relatively the same. It is best to stick with the manufacturers recommendation unless you notice premature wear(Some BMWs used to call for really high pressures in the rear to meet the load requirements and they would wear them in the middle VERY quickly), or I usually allow about 3PSI either way for personal preference. |
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