With the Avon tires, you would have to mix between 2 different kinds, but Bridgestone battalax you can get in 110/80V17 and 140/80v18. You should be able to go up a size on the rear without any problems, but don't quote me on that. The only problem i found with the Bridgestones is that they stress crack early from heavy breaking. And i'm not sure, but i believe my old 86 FZ 750 came with non-matching rim sizes. Someone correct me if i'm wrong...it does happen from time to time.