I have a set of 325/65/18 tires that I would like to put on my dad's truck. He has the standard chrome XLT 18 inch rims. Would they fit, or are the 325s too wide?

 

those are too wide of a tire. You'll need at least a 9" rim for those tires and your stock rims are 8".
you can go 305's but that's pushin it for the 8" wheels.

 

oh ok thanks for the info!

 

They are a wide tire, I have some on 10" wide rims and they fit perfect. But I think they would rub like crazy if you put them on stock rims.

 

I have an 07 lariat with stock rims and I'm running Nitto 325s on my stock rims no problem

 

You can run them on your stock rim, but you will not get a whole lot of miles out of them. When you put that wide of a tire (12 3/4") on your stock rim (8") the tire will bow to fit on that rim, causing the the tread to arch. This will cause premature wear down the center section of the tire. You could air down to improve the contact patch, but since you've already shoe-horned those balloons onto your wheels then you get into stability and safety issues. This is where people get into the love/hate of tire life. If you've wondered why somebody gets 50K on the same tires that another person gets 25K. Sure driving style, lift, type and condition of roads driven plays a factor, but putting your tires on the right wheel dimensions and keeping them properly inflated will go a long way to getting the most out of your tires.

BTW BF Goodrich recommends a rim that is 9-12" wide for a 325 tire.

Just my .02