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Discussion Starter · #1 ·
hey guys,
i was wondering what would be the best pressure for my tires. i have 215/45 17's nitto 450's. the max pressure is 44 psi.

i'm more concerned with fuel efficiency but at the same time not popping the tire. i know you calculate between 85 and 90 percent of max pressure.

i've taken my car up to 148 mph once on a whim. i wasn't at that speed for more than maybe a minute. if i inflate my tires to 38, and ever got up to top speed again would my tires pop?

i'm debating between 36, 37 and 38 psi.

thanks,
peter.
 

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i'd take 38 cold. that way when it's fully hot running it would be like 42, which is still ok. taking the car to 148 is no problem, coz' i believe the tires are z rated and it should be ok. it's the v rated tires that induced the 149mph speed limiter on our cars
 

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Tires at high speed generally expand towards the center, like a drag car's tires do on a burnout. If you want to travel at high speeds, best thing is to set the pressure slightly below what you normally would to allow for that factor, and make sure you've got the most rubber gripping the road.
 

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Not to contradict, but most people would increase their tire pressure at the race track (I.E. high speed/high heat). Anyway tire manufactorers have to test their tires at extremes to make sure they don't blow up, high pressure/high speed.
 

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Right, but don't forget that heat expands/cold contracts, so in effect high speed driving/cornering raises the air pressure inside the tire, more so than ambient temperature (which is also a factor).

The reason to slightly underinflate tires travelling at high speed is twofold. First, the heat will raise the tire pressure as we've stated, but also because of center expansion (as a drag car's tires do). The center expansion is due mostly to the centrifugal force that the tire experiences at high RPMs. Tie something on a string and swing it around......that's the same force that acts upon the center of the tire tread.

As well, because a tire may be tested at high speed to make sure it doesn't blow up, doesn't necessarily mean that that's its ideal performing conditions.
 

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At an autocross when you use street tires, you over-inflate them. I used to run 42psi on the stock michelins. :eek:

However - competition tires are a different beast. Their sidewalls are like 2-3x as stiff as street tires, and you want to under-inflate them so they can get the best grip. Grip on a properly heated-up competition tire is DISGUSTING.
 

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Straight from TireRack.com

Maximum Inflation Pressure
A tire's maximum inflation pressure is the highest "cold" inflation pressure that the tire is designed to contain. However the tire's maximum inflation pressure should only be used when called for on the vehicle's tire placard or in the vehicle's owners manual. It is also important to remember that the vehicle's recommended tire inflation pressure is always to be measured and set when the tire is "cold". Cold conditions are defined as early in the morning before the day's ambient temperature, sun's radiant heat or the heat generated while driving have caused the tire pressure to temporarily increase.

For the reasons indicated above, It is also normal to experience "hot" tire pressures that are up to 5 to 6 psi above the tire's recommended "cold" pressure during the day if the vehicle is parked in the sun or has been extensively driven. Therefore, if the vehicle's recommended "cold" inflation pressures correspond with the tire's maximum inflation pressure, it will often appear that too much tire pressure is present. However, this extra "hot" tire pressure is temporary and should NOT be bled off to return the tire pressure to within the maximum inflation pressure value branded on the tire. If the "cold" tire pressure was correctly set initially, the temporary "hot" tire pressure will have returned to the tire's maximum inflation pressure when next measured in "cold" conditions.

A tire's "maximum inflation pressure" may be different that the assigned tire pressure used to rate the tire's "maximum load". For example, while a P-metric sized standard load tire's maximum load is rated at 35 psi, many P-metric sized standard load performance and touring tires are designed to contain up to 44 psi (and are branded on their sidewalls accordingly). This additional range of inflation pressure (in this case, between 36 and 44 psi) has been provided to accommodate any unique handling, high speed and/or rolling resistance requirements determined by the tire and vehicle manufacturers. These unique tire pressures will be identified on the vehicle placard in the vehicle's owner's manual.

The tire's maximum inflation pressure is indicated in relatively small-sized print branded near the tire's bead (adjacent to the wheel) indicating the appropriate value. Because tires are global products, their maximum inflation pressure is branded on the tire in kilopascals (kPa) and pounds per square inch (psi). These values can also be found in the industry's tire load & inflation charts.
 

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Discussion Starter · #10 ·
i've set my tires at 36psi. but they feel a little soft when cornering at around 20-30mph. any thoughts?
peter.
 
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