255/55 should be fine, but you would want to replace the spare with the same size; you don't want to have a different diametre tyre on 1 corner of the car. I assume you have rotated the tyres, including the spare, to get 50000 km out of your originals. Sure have. Every 10K full rotation, including spare! Mind you, I'm doing consistent freeway driving. Gosford to Sydney return, every day of the wk. I'm going to put the yoko tyres on tomorrow. 255/55