I went 255/45/20 Toyos Proxes4. Yea you are correct what ever the second number is a % of the width that then relates to sidewall height .So I have a 10" wide tire(255dividedby 25.4 =10.0397wide/at 45% is about 4 1\2" side wall.) does that help you ? So yes a 55 has a taller sidewall than a 50