On my 2008 F350 I'm
Running 18inch rims/tires. Right now I'm running a 305/70/18 tire. What would be a better choice with more tires available to choose from?

I was thinking 16inch rim. What tore would be equal in size in a 16??

 

There's a nice calculator at

Tire Size Calculator

It'll let you enter your size, and the new diameter, and give you the equal.

Of course, it doesn't tell you about fitment, you might search the new tire size here on the forum to see if others have already done the same.

 

From what I have read, 16" rims wont fit over your trucks brakes. I think you can go to 17" at minimum.

 

On the 2005 and newer trucks, 16 inch rims will not clear the brakes. 17 inch is the smallest you can go.