Could I talk you into flipping the center caps over and taking an inside and outside measurement?
I'd want a set of silver ones for the alloy rims on the 4Runner.. and a set of black ones for our '85.
If I remember correctly the 91 4Runner's were 4 1/2 where they snap into the rim - and 5 1/4 on the outside where they meet the rest of it?
I'll measure for sure tomorrow.
