I think +/- 10% is incorrect.
Where did you get that, or is it a guess?
I think the law requires the speedometer to NOT read low (in OEM configuration, tire pressure, etc.), so the DISPLAYED speed is usually about 2% over the internally "calculated" speed.
If you are 8% high, that wiuld seem unusual, and your tire pressure and temperature might be low?
Using the hidden "Calibration" screens (with Audio power off, quickly press: Map, Map, Map, Audio (on), Audio (off), Map).
One of the screens shows the "wheel" and GPS derived speeds (100.0 kph (shown as 1000) = 62.5 mph) and the LEAF will "usually" DISPLAY about 65 mph in the dash "eyebrow".
It might also indicate if the wheel (or motor RPM) derived speed differs from the GPS derived speed.
I am not aware of any way to change the Speedometer "offset", or recalibrate the internal speed. Since the LEAF has the GPS-derived speed available, it COULD be self-calibrating, and carefully reporting 2% high. But, if your car is actually showing 8% high, then it would appear that your car is NOT self-calibrating.
Of course, running larger diameter rims or higher-profile tires would make your car go faster than indicated by the RPM-derived speed.