Modifying the stock 17/40 gear combo to a 17/42 only changes the ratio by 5%. You'll still have to determine what your particular factory speedo error is and add that to the 5% gear ratio change as well. The factory speedo errors vary by so much we can't accurately approximate it for you but I can tell you this...
...with stock gearing, my friends 99 Busa was only off by 3%, my 99 Busa was off by 5.25% and still others have found 7% to 10%. The one factor that can accurately be determined is the 5% gear ratio change the 17/42 gear combo makes from stock.
