I just threw those sizes into http://www.rimsntires.com/specspro.jsp (uses Flash so prob won't load on your mobile) and it says that the front diameter is 680mm and your current rears are 674mm. If you go to a 275x35x20, the diameter will go up to 700mm which - obviously - is a 20mm dif to the fronts so I'm gunna expand your question if you don't mind Matt.............. Does anyone know what the maximum difference between front and rear tyre diameters is on the FG ??? As I was (still am sort of) thinking of going up in profile on the rear of mine as well (18"), I went up to my closest dealer a few weeks back to ask and that was a waste of fuel coz the bloke had NFI and had no interest in trying to find out.