You can check speedometer accuracy several different ways.
Use GPS to see your current speed and compare that to your speedo reading. Make a mental note how far off your speedo is and keep that in mind when you go over the posted limit. But nobody does that, right?
No GPS? Use the mileage markers along the road and keep your speed at 60. If you travel 1 one mile in one minute by the signs and your watch, and your speedo concurs, congratulations!
Use a calculator to determine your percentage of error. (actual speed divided by indicated speed). If your speedo reads 65 and GPS says 60, you're off by about 8%. At 75 indicated, you are actually traveling 70 mph. Good enough for me.
If your custom speedo can be adjusted to match your GPS, you're golden. If not, just remember how far off you are and enjoy the ride. :moped:
Most Honda speedometers do read fast (5-8 %) which helps Honda avoid product liability lawsuits.