The 235 number is the width in millimeters.
There are 25.4mm per inch, so 235mm*(1in/25.4mm) = 9.25"
The 85 number is the aspect ratio (sidewall height/width) which represents 0.85 ratio. So 235mm*0.85 = 199.8mm.
Converting the 199.8mm height to inches 199.8mm*(1"/25.4mm) = 7.864".
Since the side wall height has a top and bottom, the overall sidewall height is 15.73".
A 16" wheel + 15.73" of sidewall height means the tire is 31.72" tall unloaded.
The larger tire size typically has more capacity, so I would go with the larger size if they will fit.
