gps speed accuracy Page 21

GPS speed is usually more honest than your car’s speedo, which in Australia is legally allowed to over-read by up to 10% + 4 km/h and must never under-read (ADR 18). In good, open-sky conditions at steady speed, dedicated GPS units are typically accurate to about ±0.5 km/h, while smartphones land around ±1 km/h. Tunnels, dense trees, urban canyons, or poor satellite geometry can nudge errors to a few km/h and add a second or so of lag. Many apps smooth readings, which looks tidy but adds more delay when you’re speeding up or slowing down. That’s why your GPS often shows a lower number than the dash—and it’s the dash that’s being polite, not the GPS being stingy. Use GPS as a guide, keep a legal buffer, and don’t expect perfection under bridges and skyscrapers.