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Old 05-30-2019, 04:27 PM   #1
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Tire pressure when +2

I read an article recently that suggested that when you increase your wheel size, but keep the same rolling diameter, e.g., going from stock 17" wheels to aftermarket 19", known as "+2," you should increase your tire pressure to make up for the smaller volume of air in the 19" tires. But how much? Is there a formula for this?

My 17" tire pressures were 29psi front and 36psi rear. What pressure should my 19" tires be?
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Old 05-30-2019, 04:35 PM   #2
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Never heard of this. Stock 18****in. Wheels have that same tire pressure. I would imagine that 19 in wheels would also have the same.
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Old 05-30-2019, 09:16 PM   #3
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Its complicated.

The key is the load index. Two different tire size combinations can have the same (or very close to the same) load index and therefore, will have similar tire pressures. That is what Porsche did for the 17 and 18 inch tires on the Boxster. If the load index changes between the old and new tires, then the tire pressure is likely to change. This is the general concept, but of course, its actually more complicated than that.

This paper from Toyo tries to explain it and gives an example for a +1 fitment using a different type of tire and different sizes. In this case, the load index changes and so the tire pressures change. Using the example as a guide, this should help you to determine the best pressures for your exact situation:

https://toyotires-1524598101.netdna-ssl.com/media/1541/application_of_load_inflation_tables_20151020.pdf
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Last edited by thstone; 05-30-2019 at 09:22 PM.
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Old 05-30-2019, 09:28 PM   #4
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Off the top off my head there is Boyle’s gas law:

P1*V1=P2*V2 @constant temperature

You can see that it will give you a higher pressure if the volume is decreased. But is this really what correction pressure is necessary for your tire? I could not say myself.

You need: the volume of your present tire; recommended tire pressure; volume of your 19. You can then solve for the new pressure that will maintain the same pressure-volume system. It won’t be easy to accurately calculate/measure these volumes. Perhaps solid modelling might get you close.

Check the result against what would make sense, but whether this what your tire actually needs as a base pressure for tire load is another story.

A quality tire specialty shop should be able to give you the right guidance on this
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Old 05-30-2019, 10:41 PM   #5
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Oh boy, tire talk, almost like and oil thread. I run big 19’s 8.5 up front and 10.5 rear. I run my fronts at about 28 and 34 cold on street, less on the track. I look primarily for tire wear patterns. This is a little trickier on the Boxster because the rear wheels have a relatively significant negative camber overall. I just look for even tire wear across the entire tire without sidewall scuffs. Most cars I run recommended pressures and leave then as they are. I’m always noodling with the P-Car pressures to get the most out of the tires, both grip and wear wise.
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Old 05-31-2019, 09:21 AM   #6
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Quote:
Originally Posted by Auf los! View Post
I read an article recently that suggested that when you increase your wheel size, but keep the same rolling diameter, e.g., going from stock 17" wheels to aftermarket 19", known as "+2," you should increase your tire pressure to make up for the smaller volume of air in the 19" tires. But how much? Is there a formula for this?

My 17" tire pressures were 29psi front and 36psi rear. What pressure should my 19" tires be?

You should use tires as close as possible to the 986 oe height.

I would see what psi is recommended for late model boxsters with similar size tires.

Compare weight of your car to the boxster you are copying 19" tire sizes from & start with that psi setting. You will want to lower the psi since your car should be lighter than the 981's but it will take time to get it right, better too much psi than too low.
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