X-axis is actual vehicle speed as reported from a GPS device or logging using OBD-II protocol. Speedometer shows 6-8% higher e.g. 107 km/h instead of actual 100 km/h.
By the way, tachometer indication seems accurate, this one is driven by square-wave signal generated by the ECU.
Might also apply to other models (same final drive ratio, wheel size)! For gearbox information (gear ratios) see this page: Transmission – 6MT