Ok, I have 18's. My dad keeps telling me to put 40 in them. When I go to wal-fart they always do like 32.5. What's an appropriate amount? I don't want to over fill them and I don't want them to go flat.

 

I don't like to see Outer Wear on my Tires and want to Maximize my Gas Mileage. So I like to inflate towards my Max. PSI of 51psi. I keep MY 18's at between 40-42 psi.
I Suggest you inflate toward the high end of the Max. PSI indicated on the sidewall of your tires. Just my 2¢ YMMV

Don't go by Wally World:

Walmart Tire and Lube busted off my TPMS

https://www.scionlife.com/forums/off...-232701/page4/

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No matter the size, or profile of tire, you place the factory recommended amount of air in the tire...the recommended amount is listed on the placard in the door jamb.....you can get away with running 2-4 psi over the recommended amount,
but any more any you will prematurely wear out your tires...as in a tire you might have got 4 years out of will wear out in 1 year or less.....

I run 36psi front, 33 psi rear on 215/40-18 front tires, 225/40 rear tires...

 

I run at least 40 psi for the past year My tires wear even and I get all around better performance When I run what's on the car it feels like my tires are under inflated