I got a SpeedoDRD speedometer corrector. It is the smallest I could find, and I have used this product on 2 different bikes (I sold the 1st one on the bike, so a bought another for my vtx1300r). Once I corrected the speedo, following the instructions, it was perfect. BUT, just so you know, Honda engineers seem to have made the speedo off by a specific %, but made the odometer accurate. What this means is, if you alter the speedometer by using a healer, you will make the odometer off.
For me, the speed is more important.
My odometer is now off by -4%. So when I have driven 100 miles on the odometer, I have actually driven 104 miles. So, when doing my mpg calculation, I just multiply miles by 1.04 before dividing it by gallons of fuel filled at the pump.
Speedometers and Odometers are off by percentages, so the faster you go, the more it will be off.