You may of bought tires not recommended for your application. As said what tires did you buy?
Well, they are passenger car touring tires: General Altimax RT 43's. 225/60 R18. What U know bout them? I've had 2 other sets of these on other cars & they are great tires.
Those Generals are rated at 1,794lb/tire at max psi of 51.
The OEM tires, Firestone FR710s, are rated at 1,709lb/tire at 44psi.
The new tire set is rated for an extra 340lbs at a higher pressure, meaning they have a thicker/stiffer sidewall to accomplish that.
I'd suggest starting at what it says on the door and let the tires break in a little before going any lower.
OE tire [I believe]
https://www.tirerack.com/tires/tire...6&autoModel=300C&autoModClar=RWDYour tire
https://www.tirerack.com/tires/tires.jsp?tireMake=General&tireModel=AltiMAX+RT43+%28H-+or+V-Speed+Rated%29&partnum=26HR8AMRT43&vehicleSearch=false&fromCompare1=yes
