Mine's off by quite a bit and I had looked into this as well. Supposedly there's a method for adjusting your cable speedo. I found some instructions on it.. as soon as I can pull them out I'll post it up, you can make your own call.
[edit]
Found it:
1-7 (deals with removing the instrument cluster)
8. Install a square drive bit into a drill. (Most drill accessory kits have them. There are 2 or 3 different sizes. Find the size that will fit into the speedometer cable coupling on the back of the speedometer.) I used a cordless Dewalt drill. Make sure that you use a battery with a good charge. What you want is a constant speed reference. A corded drill might even work better. Might turn faster giving better resolution. My Dewalt would only register about 35 mph.
9. Hold the speedometer on the drill and turn the drill on full speed and read the mph indicated. Mine indicated 35 mph.
10. If your speedometer reads faster than it should, like mine, take the indicated 35 mph and divide by the percentage that it was off (7%) to get the new calibrated speed of 32.5 mph. 35/1.07=32.5 If your speed was slower you would multiply instead of dividing.
11. Now look at the back of the speedometer. There is a small spring coiled around the speedometer shaft. This coil is held by a small bracket
that is crimped to hold the spring. If you need to lower your calibrated speed as I did you need to bend the bracket in the direction to tighten the spring (Moving it closer to the odometer wheels). It doesn't take much. I used a pair of needle nosed pliers. Make sure you do not leverage off of the round bell that is attached to the speedometer shaft. You do not want to put any pressure on the shaft. Make sure that you do not hold onto the speedometer face or touch the needle.
12. After the adjustment, put the speedometer on the drill again and check the speed. Keep adjusting as necessary.
13. When the speedometer reads the correct calibrated speed make sure that the small piece of spring that extends past the crimp on the small bracket is not touching the odometer wheels. Bend over if necessary.
14. Reassemble the speedometer in the reverse order. Install on bike.
15. Test drive.