Mustang Ecoboost Forum - View Single Post - tire psi question, need help on recommended tire PSI
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post #2 of Old 05-12-2018, 07:41 AM
Uncle Meat
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Originally Posted by ecostang2015 View Post
So I replaced the 255/40r19 Pirellis with the 96w rating with Cooper 255/40r19 with the 100W XL tires, the tire shop i think overinflated the tires. one of the tires read 40 PSI. I know my stock tires were recommended at 32 PSI cold. Do you think the Cooper's being XL load range 100W that the correct tire pressure is 40 PSI, or is that too high? I can't get reliable information because the 96W and 100W XL would probably have different PSI recommendations, right ? Then I read, if you under-inflated the tires, they go bad fast, over-inflate them, they get punctures easier. Tires are turning out to be a headache.
Relax and take a deep breath... You went from a 96W rated tire to one with a rating of 100W. There's not a whole lot of difference between those two ratings. What that specification is referring to is the load range, or more accurately as the maximum Load Index rating at maximum PSI. So your old stock tires with the 96W rating were certified to support a weight of 1565lbs per tire at maximum PSI. So 4 tires at maximum PSI times 1565 equals a total of 6260 Lbs. That's the maximum weight those tires were rated at if you pump them up to the max. You'd have to put a lot of luggage in the trunk to get anywhere near that heavy so there's no need to run them at their maximum PSI.

So the rule of thumb when replacing tires is never go with a tire that's rated lower than the stock tires which came with the car. Those engineers at Ford didn't go to college and incur all that debt for nothing you know. They didn't just arbitrarily pick "96W" out of thin air for the stock tires. Pun intended. ;-) You did the right thing by picking a tire that matches or exceeded the Load Index of the stock tires. 100W rated tires can support 1764 Lbs per tire at maximum PSI. 4 X 1724 Lbs = 7056 Lbs!

So now for the crust of the biscuit as the say, what does the Load Index have to do with PSI? Nothing unless you plan on filling the car with a few thousand pounds of cargo. So what pressure should you run those new 100W tires at? Look at the door jam on the drivers side of the car. I'll bet it says 32 PSI. Stick with the recommended PSI which is based on the overall weight of the car and driver. Just because your new tires have a higher rating doesn't mean you need to run them at a different PSI.

Drop them to 32 PSI and rotate them as recommended and they will serve you well.


2016 Ecoboost Mustang
2009 C6ZO6 - Katech Stage 2 #115
2005 Mustang V6 (Sold)
2003 Terminator (Sold)
2002 Mustang GT (Sold)

Last edited by Uncle Meat; 05-12-2018 at 05:22 PM.
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