There is a speedo gear change that is a direct bolt-on and it will give you a true reading with the 17" wheels, I just can't remember the model/type, it might have been a speedo gear from a FZR etc. I know once you get the correct model/type you can purchase it on E-Bay etc.
There is a difference for sure. Like Damon said there is a gear set from another bike that fixes the issue. Dannymax has that info, I believe. I'm going to a Koso gauge setup partially for that reason. I got a good deal(tks to Mark) on a never installed unit. It came with the adapter to use the OEM cable, and a temp sender.
Appreciate the info everyone. I ordered a speedo drive gear for a YZF600R (Thundercat in Europe) from Ebay (for not a lot of money, always a good thing).