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Where Does the Speed Data Come From?
The RoadScout RST is plug-and-play device that connects to the vehicle’s OBD-II port. It can read OEM parameters such as odometer, fuel level, and speed directly from the car’s systems.
Where does the speed data come from?
- In most vehicles, the speed data is taken from the Engine Control Unit (ECU).
- The ECU measures speed before it is adjusted for display on the dashboard speedometer.
- This means the RoadScout reports the “raw” speed signal, not the slightly inflated version you see on the speedo.
Why is the dashboard speed different?
- Under Australian Design Rule (ADR 18/03), speedometers must never show a speed lower than the actual speed.
- They are allowed to over-read by up to 10% + 4 km/h.
- Since 2006, manufacturers have calibrated speedometers to overestimate slightly (often ~5%) for safety.
- Example: If you’re driving at a true 100 km/h, your speedometer might show 105 km/h.
How does this compare to GPS speed?
- GPS speed is calculated from satellite positioning and is usually very accurate on straight, consistent roads.
- Because the RoadScout reads ECU data (before speedo adjustments), its reported speed is typically closer to GPS speed than what you see on the dashboard.
Summary
- Speed data = ECU data (raw, before speedo adjustment).
- Dashboard speed = ECU data + manufacturer’s safety margin (slightly higher).
- GPS speed ≈ ECU data, so RoadScout readings align more closely with GPS than with the dashboard.