---
title: Where Does the Speed Data Come From?
description: "GPS Speed\nECU Speed"
---

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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.

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