Tires: Change out tire size, tire placard, ford engineers

Question
I have a 2006 Ford Mustang, can I replace the current p235/50zr18 tires with a p235/40zr18 sized tire all the way around?

Answer
Jon,

Let's start with the basics:

On every vehicle sold in the US, there is a sticker – commonly called the tire placard - that lists the original tire size and the proper pressure for that size.  The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.

BTW, it doesn't matter who makes the tire or what pressure is listed on the tire's sidewall, if the tire size is the same as the placard, then the pressure listed on placard is also appropriate.

The reason I want to start there is to determine what the Ford engineers felt they needed for load carrying capacity.  That is a combination of the tire size and the inflation pressure.  That would be the starting point for any further discussion.

I also looked up your vehicle in Tire Guides - a book that summarizes what vehicle tire placards say.  But the book lists quite a few different versions of 2006 Mustangs, but none of them came with 18" tires all around.  So I need to be sure the book is correct - and if you'll tell me what the vehicle tire placard says, I can take it from there.