It should not be off by that much...
I'd have someone pace you, either in a car or another bike.
Go 50mph on your speedo, and have them match your speed. Then you can find out what their speedo said.
Do it at a couple different speeds.
If it is off, have the dealer look at it, cause that's not good
I had to do this in my TransAM after I changed the wheel size...
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2002 Green 6R
1986 Gixxer 7/11