In the metric tire sizing system the second set of numbers is an aspect ratio of the width (percentage of width), this is a common mistake many people make as it is possible to have a taller 50 series tire then a 55 series tire. For example a 285/50r20 will have more sidewall then your 235/55r20, although it is a 31” tire. The .1” in diameter increase will have no improvement on ride and the tire will look funny since it will be so narrow. Stick with the stock sized tire.
In order to get more sidewall but keep approx. the same width you would need to go down in rim size from 20 to 19 or even 18. The online tire size conversion charts are handy for figuring out dia. and width measurements.
There is no "more" cushion the sidewalls are the same. Below is another attemp to explain.
The difference in side wall height is negligible from the stock size and 235/55r20. What I was trying to say in my previous post is that the sidewall is the SAME between 235/55r20 and 255/50r20. You are NOT getting "more" sidewall with a 235/55 then a 255/50. The 235/55 side wall is .875 mm larger then a 255/50 depending on tire manufacturer. Now compare different brands of tire and you discover that manufacturing tolerances will be greater then that. If you want MORE side wall you need to go to a larger aspect ratio with same width or the same aspect ratio with a wider tire. See description below. For reference a 235 width tire is recommended up to 8-8.5" max rim width. So technically you could fit that tire on your stock wills but it will be stretched to that rim size and will offer less rim protection from curbs/rocks etc.
Aspect Ratio is the ratio of the height of the tire's cross-section to its width. The two-digit number after the slash mark in a tire size is the aspect ratio. For example, in a size P215/65 R15 tire, the 65 means that the height is equal to 65% of the tire's width. The bigger the aspect ratio, the bigger the tire's sidewall will be.
Rovingryan, thanks very much for your explanations. Now I believe I understand it. I was not aware of the aspect ratio. So, all in all, I need to fit 255 55 r20, which seems to fit as per buba post (thanks for that buba). Going to 275 55r20 will be better but maybe it will requiere mods. Thanks