Many countries have standards governing speedometer accuracy. From what I've seen, that standard is usually +/- 10%. You are WELL within that tolerance. If you want to check your speedometer accuracy, compare it to a GPS speed. My SI is dead on up to 70. Then it reads about 2% fast up to 120mph, then gets accurate again.
To see if your speedo is correct, you could also use
this info. For me, I will set my cruise control to a given speed, like 60 or 70, and then use the mile markers on the highway. Using a stopwatch, I will time how long it takes me to go from a mile marker to the next marker. If traffic allows, and for the best results, I'll use two or three mile markers for a longer amount of time. Do the math to see what your speed is and there you go. You will now see if your speed is correct. If your speedo is registering correctly, then you know your odometer is reading correctly as well.