A 295/40 20 means the tire section width is 295mm. The 40 means the aspect ration or (Side Wall) is 40% of 295mm or 118mm divide by 25.4 and you get 4.64". You would then subtract the lip height if you want viewable rubber. Hopefully this will help. Maybe one of our tire and wheel experts will correct me if I blew it. This is my understanding.
Good Luck!
