On bikes with chain drive, if you swap out either of the sprockets, it changes the speedometer reading. That's where the Speed-O-Healer is needed the most. But then there's the guy that "must" have his sruff perfect!
Not if speedo is driven by the speed of the front wheel. Changing sprockets will change engine RPM for the same speed. The amount of air in the rear tire can also effect the speedo input from the front tire. Example, the local Honda shop owner saw the rear tire on my 900. He suggested I add another 3 pounds of pressure to correct tread wear. I did and noticed at 4000 RPM the speedo had raised a couple of miles per hour.
A friend of mine didn't look too closely at his speedo vs. GPS. He said during a recent run that he tucked in and rolled on during an open stretch... he said just glancing at his speedo it looked close to buring it? Checking the GPS he maybe was close
Some people just like to have an accurate speedometer. I know I wish MINE were in all my vehicles.
Don't worry about it. Drive 5 mph (indicated) over the posted speed limit, and you're OK still. Just forget about the difference, and don't get speeding tickets.