GPS measures speed very accurately, from the Doppler shift of the satellite signals, but a phone only reports it about once a second. The accelerometer reports a hundred times a second, but on its own it drifts within seconds.
So the accelerometer decides when you launched, to a few milliseconds, and the moment you passed 60 is found between two GPS readings: the shape of the speed curve comes from the accelerometer, its size from the GPS.
A phone’s GPS speed arrives late: the reading reaches the page after it was taken, and the chip smooths speed with its own filter, which makes it trail the truth by a few hundred milliseconds. Left in, every time would be that much slower.
The first part is removed by using the time each reading was taken, not when it arrived. The second is measured from your own run: the GPS speeds are slid in time against the accelerometer until they line up, and the times are corrected by that amount. Your phone’s average delay is remembered, for runs too short to measure it.
The phone can sit at any angle. It works out which way is down from gravity, and which way is forward from the launch itself.
0–30 and 0–60 mph, 0–100 km/h, 0–100 mph, the quarter mile with its trap speed, and 60–0 braking distance when you brake to a stop. One-foot rollout is an option, to match the times magazines publish.
Within about a tenth of a second on a typical phone, on simulated drives with real GPS behaviour. A dedicated 10 Hz GPS receiver does better still; a phone’s once-a-second GPS is the limit here.
© 2026 Speed
