How accurate is the iPhone GPS speedometer?

Speedometer uses the iPhone's built-in GPS, which is typically accurate to within 1–3 km/h (or 1–2 mph) under clear sky. Accuracy depends on satellite signal strength — performance is best in open areas with an unobstructed view of the sky, and slightly reduced in tunnels, dense urban canyons, or under heavy tree cover. Speedometer also reports the current GPS accuracy band so you can see when conditions are degraded.