Tire pressure??

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During my holiday travels I checked my tire pressure. They were 5psi below the max psi printed on the tire (35psi). I filled them to 35 and noticed how much more 'feedback' I got from the road. Then I looked at the sticker inside the driver's side door and it recommends 29psi. I still have the origional stock tires. Which is correct, the vehicle manfacturer's recommended pressure, or the max the tire manfacturer recommends?
 
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You want to go by the label by the driver side door. Tire manufacturers usually do not give a recomended inflation number because one model of tire might go on 10 different car models of varying weight/capabilities etc. The number listed on the tire is max psi, which incidently is usually above the 35 psi you listed? In any even, I usually go with about 32 psi cold when my recomended psi is 30; 32 gives better control/ride IMO. [Cheers!]
 
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Also keep in mind that in the cold winter temps you'll "lose" air pressure. Say you inflated them when the outside air temp was 60*F and it's 0*F this morning you'll have about 6 psi less in the tire. So if you inflated the tire to 35 lbs when it was 60* it's now about 29 lbs at 0*. Also your tires should be inflated "cold" before you run them and they heat up. I have a tire pump that uses my cig lighter for power and do it in my driveway first thing in the morning before I drive the vehicle. Whimsey
 
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35 max. is the normal passenger tire pressure rating. If you look a bit more, you'll see where the sidewall also says to follow the placard in the vehicle for proper pressure. As long as you keep the same size passenger tires on your car, regardless of tire manufacturer, this pressure will be correct. Remember, radial tires are supposed to have a bulge at the bottom, causing them to look a bit low on air.
 
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I notice a lot of people look at the sidewall and assume that the pressure listed is the "correct" pressure. The psi listed on the sidewall is the <b>max cold pressure</b> . You usually don't want to inflate your tires to the max cold pressure. I use the 75% rule. That is, 75% of the max cold pressure.
 
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If I rememeber, the Ford Explorer tire psi was set low to reduce the chance of rollover, not for comfort purposes. As for the 75% rule, this isn't always safe either. For example, this would place a Ford Explorer with passenger tires at 26.25 psi. This number is lower than the original number Ford placed on these vehicles, 28 psi. My '86 Olds Cutlass called for 35 psi in the tires. The max cold psi for these passenger tires was 35 psi. Once again, 75% would leave me at 26.25 psi. This could create a very serious saftey issue. Underinflation is significantly more dangerous than overinflation (inflating to max cold pressure). The vehicle manufacturers have engineers that are payed very well to come up with the number they post in the owners manual and place on the door jam sticker regarding proper tire pressure & size. These engineers don't try to tell me how to put out a fire or treat a heart attack. Why should I tell them they did their job wrong when I have no idea how they do it.
 
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The Michelin tire book that came with the truck said to add 10 psi for speeds over 65 mph, so I end with a range of something like 40 psi in the rear with no load and driving under 65 mph, to 80 psi with a max load and driving under 65 mph. I need to have less than a max load for speeds over 65 mph. Looking at the big stuff being towed at 75 mph to 85 mph on the highways I guess people use much better highway tires than Michelins.
 
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I don't care for the 75% thing either, both vehicles I drive now have recommended tire pressures that are in line with the maximum pressure on the tire. I.E. Honda Odyssey recommends 35 or 36, and so does our PT Cruiser. On the Ford Explorer issue, I don't know what the deciding factor was, but I do remember reading that Firestone recommended a higher tire pressure than Ford ended up using.....
 
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The car manufacturer's tire pressure recommendations are usually on the low side, at least in the case of passenger cars the recommendation is geared at comfort. I find 5 psi more improve handling considerably on my car. I run my 205/55/16 usually around 36-38 psi. If the speed limits were higher, I'd inflate them a bit more. Don't forget that some cars require different pressure in front and rear. The VW Corrado comes to mind...
 
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quote:
Originally posted by medic: The vehicle manufacturers have engineers that are payed very well to come up with the number they post in the owners manual and place on the door jam sticker regarding proper tire pressure & size. These engineers don't try to tell me how to put out a fire or treat a heart attack. Why should I tell them they did their job wrong when I have no idea how they do it.
If we knew what tire pressures those engineers actually used in their own cars and for what type of driving, we would have the advantage of knowing what they actually thought. We don't. Generally speaking a few more psi improves handling and tire life. Door jamb pressure gives near maximum comfort if you are a conservative driver.
 
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Another thing someone hinted on is that the numbers on the inside of the door are COLD psi. So if you drive your car to the gas station etc... then you need to add more. I have found 35 for the front and 33 for the rear gives very even tire wear. I did this for all my customers and told them about setting tire pressure cold vs warm.
 
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Just a thought. I seem to get better tread wear when I inflate the tires on my truck to their recommended rating on the tire. It may be a correct setting for this particular vehicle and I just don't know it. [I dont know]
 
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Trucks are a bit different. Most use a light truck tire, some rated up to 80 psi depending on load range. Trucks are different because of the variable weight that may be carried. A heavier load requires more pressure. Therefore, the vehicle manufacturer usually sets the tire pressure at the maximum for the rear tires. Some people, like me, rarely have a fully loaded truck and don't need all of that pressure all of the time. This high pressure may cause the tires to wear in the middle of the tread, and the outside tread never touches the road. This can also cause the rear of the truck to bounce, like a basketball, completely losing contact with the road at times. I typically run my rear tires in the 20-25 psi range and have been doing this for over 100,000 miles. I just have to remember to increase the pressure if I am going to haul anything.
 
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I find that a few psi above the vehicle recommendations gives more control, better mpg and tire mileage. Sometimes their recommendations are compromises for a softer ride - remember the Ford Explorer? If you run higher pressures and the tire wears more in the middle then you know you are too high. Or if the ride is too bone jarring. John
 
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Yes, whenever car or tire manufacturers are specifying tire pressure, it is a "cold" pressure. The maximum tire pressure specified on the tire is exactly that, a maximum. It is not a "recommended" tire pressure, unless conditions that the tire is experiencing calls for the maximum. If you were an anal retentive person, you would be adjusting your tire pressures upward or downward depending on whether you were traveling with a full load of passengers, lots of luggage or other load, etc. In the real world, What I do is see what the auto manufacturer recommends, see what the maximum is on the tire, and then split the difference. On the cars I drive, I typically run about 35 psi. And tire pressures do drop on their own in winter due to the fact that gas (air) contracts when chilled. So, if you last checked tires in warmer weather, check again when it gets cold. And then check again when it warms up again. (Of course, if you are the aforementioned anal retentive, you would be checking every two weeks in any event.) Years ago, I got tired of going to the service station for air, and I purchased a home compressor and good tire gauge. As a result I really can check tires totally "cold". One of my better decisions.
 
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quote:
If you were an anal retentive person, you would be adjusting your tire pressures upward or downward depending on whether you were traveling with a full load of passengers, lots of luggage or other load, etc.
My Audi shows a little schematic inside the gas flap. It shows the car with up to two (skinny) people, 4 (skinny) people, and suitcases (medium size), with tire presssure suggestions for each of those scenarios. Anal enough? [Wink]
 
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Wow, that's handy! Generally you can play with the pressures anywhere between the vehicle recommended PSI and the maximum stated on the tire, but not above or below this. You should know how the changes you make affect the vehicle before messing around though... The Michelin guide 1sttruck mentioned probably *recommends* 10 PSI more for continuous high speed driving, and that isn't because their tires suck, it's because that's probably a good suggestion for everyone doing that. A tire is rated for XX load and X speed rating only at maximum pressure. Whenever you're pushing the capacities of the tires you should increase the pressure.
 

blupupher

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If your really anal about it, you should find out you vehicles actual curb weight (with pass and cargo) and find the front to rear split, then find your Max pressure/weight on the tire and figure it from there. So if you had a 4000 lbs car split 50/50 front to rear weight distribution and you tire has a max pressure of 35 psi/1000 lbs rating, then you would be at 35 psi per tire. a 3000 lb car would be 26.25 psi
 
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