Proper Air pressure for 255/40/18 on 18x9 wheel - TCCoA Forums
 
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post #1 of 8 (permalink) Old 02-28-2015, 12:13 AM Thread Starter
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Proper Air pressure for 255/40/18 on 18x9 wheel

Hey guys,
Looking at getting the black speedstars from Custom Wheel Shop along with some General GMax AS-03's in 255/40/18 (same diameter as the 255/50/16's on it now and speedo is accurate). I had the Gmax tires on my 1999 Mustang GT and loved them, but on my Mustang they were in the stock size so air pressure was easy.

Anyone running 18x9's care to chime in with what you run for air pressure and how it wears?

Thanks!
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post #2 of 8 (permalink) Old 02-28-2015, 07:49 AM
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Since they have a load index of 99 which is higher than the OEM tire, you can actually go lower with the air pressure. I cannot find the guide I used to use that showed load index by tire size and how much each tire would support by PSI.

Something similar to http://toyotires.com/tires-101/tire-...flation-tables

page A8: http://toyotires.com/sites/default/f...-LT_102913.pdf

However they show a 255/40/18 being load index of 95, not 99.

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post #3 of 8 (permalink) Old 02-28-2015, 08:41 AM
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for daily driving and street use tires put the tires at 80% of their max pressure rating

so if its 50psi max put it at 40, if its 44 max put it at 35

If you have a drag tire, start at 20psi and adjust accordingly based on contact patch

thats my take from my own experiences

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post #4 of 8 (permalink) Old 03-12-2015, 09:30 PM Thread Starter
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Quote:
Originally Posted by decipha View Post
for daily driving and street use tires put the tires at 80% of their max pressure rating

so if its 50psi max put it at 40, if its 44 max put it at 35

If you have a drag tire, start at 20psi and adjust accordingly based on contact patch

thats my take from my own experiences
How does running at 80% max do on tire wear? I'm not concerned with racing/drag as the car is a street car. I may take corners quickly, but no racing.
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post #5 of 8 (permalink) Old 03-14-2015, 10:22 AM
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You might want to do the chalk test.

Start with a good alignment and a ballpark guess tire pressure. Use chalk to draw a line across the width of the tread. Drive it, and see how the chalk wears. Try different pressures until the chalk wears evenly. That's how your tires will wear.

1996 Thunderbird LX (4.6), previous daily driver

1995 Mustang GT (5.0), daily driver
1995 GMC C2500 (5.7), alternate daily driver
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post #6 of 8 (permalink) Old 03-17-2015, 11:13 PM Thread Starter
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Quote:
Originally Posted by Urambo Tauro View Post
You might want to do the chalk test.

Start with a good alignment and a ballpark guess tire pressure. Use chalk to draw a line across the width of the tread. Drive it, and see how the chalk wears. Try different pressures until the chalk wears evenly. That's how your tires will wear.
Thank you! I will try that.
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post #7 of 8 (permalink) Old 03-17-2015, 11:30 PM
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+1 on the chalk and just watch the wear. Pick up a $5 tread depth checker and measure inside, center and outside at every oil change. Keep track of it and adjust as needed to keep them wearing evenly.

BTW this is also a great way to catch problems with your alignment before something out of wack totally kill your tires.

Jim Stainer
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post #8 of 8 (permalink) Old 03-18-2015, 03:30 PM Thread Starter
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That's a good idea. I'll pick one up when I get my new wheels/tires.
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