Speedos are notorious for being inaccurate always overstating your speed. The most accurate method of checking is by using a GPS system as these have <1% error
They are also not linear in their inaccuracy, e.g. in my volvo teh following happens
Actual speed(GPS) vs Speedo
30 32
40 44
50 53
60 62
70 77
80 87
Cough Cough plus some
