I am looking to get some Michelin PS2's since my front 2 tyres are worn. I was looking on mytyres.co.uk and I can't understand the difference between the 225/45 ZR17 91Y with rim protection ridge, N3 and the 225/45 ZR17 91Y with rim protection ridge, RR. What is the difference between RR and N3? Which ones would I need?