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Old 11-06-2008, 07:05 AM   #1
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tire pressure question

I just installed 18 inch rims on my Focus. I have no experience with such low profile tires. Should I still run Fords recomended tire pressure settings? I am using 215 35 R18 Nitto tires


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Old 11-06-2008, 07:44 AM   #2
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I always run 2-5psi less than the tire max spec. If your new tire spec is 50-60psi, the Ford 34psi spec is too low. You will get too much flex in the tire and wear on the edges. You live in a hot climate, so I would go somewhere in the range of 5-8psi less than max when the car has been sitting since the pressure will increase as the tires heat up.
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Old 11-06-2008, 09:56 AM   #3
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Originally Posted by FucusFace View Post
I always run 2-5psi less than the tire max spec. If your new tire spec is 50-60psi, the Ford 34psi spec is too low. You will get too much flex in the tire and wear on the edges. You live in a hot climate, so I would go somewhere in the range of 5-8psi less than max when the car has been sitting since the pressure will increase as the tires heat up.
I disagree, the maximum pressure on the tire sidewall is just that, its the absolute maximum that can be used in the tire. It has NOTHING to do with what is appropriate to actually use. Its a really bad starting point to suggest!

Start with the manufacturers recommended spec for the specific tire. Adjust upwards or even downward from there. Tire manufacturers suggest that when plus sizing has been used an increase of two pounds per plus/size increase is the most that is usually needed. For the street its unlikely you'll need or want more than 4 or so PSI increase to maintain a balance between ride comfort and handling. Even the autocross boys won't use much more than 40-42 PSI. They simply find the sweet spot where the sidewalls and tire shoulder no longer roll over and a consistent temperature is maintained across the full width of the tread (that means optimal surface contact has been achieved). The best way to do this is with a pyrometer.

Too much pressure reduces traction, is responsible for blowouts and tire damage (sidewall ply separation), results in longer stopping distances, and causes the suspension (the dampers) to work much harder than they need to. High pressure tires act like an overfilled basketball and bounce excessively causing the springs to become overly active and causing the dampers to work overtime trying to keep everything in contact with the ground.

Start with the recommended pressure and adjust it in one or two pound increments until a good balance is reached.
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Old 11-07-2008, 12:18 AM   #4
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Geezer is 110% right, NEVER run your tires at max psi, or even anywhere near that number, ITS JUST NOT SAFE. The air pressure in the tires are set to the car, not the tire, because all tires do not have the same max psi rating, where this set might be max of 55psi, the next set might be 45psi, so the max psi will never be the same unless you buy the same tires all the time.

i would run about 36-38 psi in those tires, slight add in pressure will give you a little more protection for your rims, and give you a smooth enough ride a well. i say start around 36 and adjust as you need too but never run anywhere near the max psi rating
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