Hi I have a 2014 forester xt 

currently on 255/55/18 road tyre I’m wanting a larger side wall all terrain without rubbing. 
would a 255/65/18?? I know it’s only 10mm but just don’t want to buy them and regret it. Bfgoodrich I’m wanting. 
regards jay

Wotcha and welcome - Not sure about the rolling radius impact that my cause your diff to suffer. Would you consider going down a wheel size then the tyre wall would be the size you require with no risk of rubbing

A 255/60 x 17 tyre is exactly the same size if you think about going 17".   A 255/65 x 17 tyre would increase the size by 3% = 25mm diameter.

Might be worth looking at the tyre pressure info chart on the door (or in the handbook), maybe the winter tyre sizes might give you some ideas.

Going up to 65 aspect on 18" (+7% overall) increases the diameter by 51 mm, so an increase in sidewall depth of 25.5mm, by getting out your trusty measuring device and going all around the tyres on all steering angles including full lock, don't forget to allow for spring compression over undulations, you'll be able to see if there is any likelihood of fouling.

These are all from the tyre size calculator i use.

What such an increase would do to your transmission i don't know, but before you fit tyre/wheels of a size not listed in the handbook (incl staying at or above stated speed and load ratings) run it by your insurer, better find out before you spend £hundreds if they're happy about this.

If its ride quality and/or noise thats getting to you, a change of tyre make and model can make a world of difference, i've just dumped the summer set of Fuldas which were hardly worn and a set of new Federals are now on the summer wheels in my garage, for both of the above reasons, the Fuldas offered fantastic wet grip but at an unacceptable cost of noise and harsh ride.

Actually, rethinking this, are you sure its 255's you're on, i would have though 215/225's, cos 255's are Hilux width and they are massive tyres?