What is the difference between speed and velocity?
Speed is a scalar quantity that measures how fast an object moves, regardless of direction. Velocity is a vector quantity that includes both speed and direction. A car driving 60 mph north and a car driving 60 mph south have the same speed but different velocities. In everyday driving and most calculator use, the two terms are used interchangeably, but in physics problems involving forces, acceleration, or relative motion, the distinction matters.
What is the difference between average and instantaneous speed?
Average speed is total distance divided by total time over a trip, so a 300 mi trip in 5 h gives 60 mph even if you actually drove 75 mph for stretches and stopped for fuel. Instantaneous speed is the speed at one moment in time, like the value your speedometer shows right now. This calculator returns average speed because it uses the full distance and full duration; it does not capture variation along the way.
How do I convert between mph, km/h, and m/s?
The exact conversion factor defined by NIST and the BIPM is 1 mile per hour = 1.609344 kilometres per hour. To convert km/h to mph, divide by 1.609344 (or multiply by 0.621371). To convert km/h to m/s, divide by 3.6, because 1 km = 1000 m and 1 h = 3600 s. So 100 km/h ÷ 3.6 ≈ 27.78 m/s, and 100 km/h ÷ 1.609344 ≈ 62.14 mph. The factor 1 m/s ≈ 2.237 mph follows from combining the two.
How accurate is the speed reading from a phone or GPS?
Consumer GPS receivers typically report speed accurate to within roughly 0.1 m/s (about 0.2 mph) under open sky once they have a steady fix, because they derive speed from the Doppler shift of the satellite signals rather than from differencing positions. Accuracy degrades in urban canyons, tunnels, dense forest, or when the receiver has just acquired a fix. For most driving, cycling, and running uses the GPS reading is more accurate than a typical analogue speedometer.
Why does my car speedometer read slightly higher than my actual speed?
Regulators in most countries require that speedometers never indicate less than the true speed, so manufacturers deliberately calibrate them to overread by a small margin. UNECE Regulation 39, which is followed across the EU, UK, Japan, Australia and many other markets, allows the indicated speed to be up to 10% plus 4 km/h higher than the true speed but never lower. In the US, NHTSA and SAE J1226 follow a similar principle. Larger tyres, low pressure, or aftermarket wheels can shift the reading further, which is why a GPS speed is usually a few percent below the dashboard value.
