So I have 20" rockstars with 325/50r20 toyo proxes st's and apparently that's a discontinued size but I love how it goes with my wheels and truck.I went to the local tire shop and he said a 305/50r20 is only gonna be a inch or so narrower but same height.is this true if not what's the closest tire size to a 325/50r20?

 

https://www.f150online.com/forums/sh...d.php?t=374022
A few tire size calculators here. Some compare side by side, others one at a time and do the math. You'll quickly find a favorite. Hope that helps!