2026-05-04 · 9 min read
Why does my car's speedometer read different from GPS?
Factory speedometers are calibrated to read 2 to 5 mph high. Here is the regulation, the math, and why a GPS reading is the honest one.
Blog
Articles, deep dives, calibration notes.
The web speedometer is small. The questions around it are not. These posts cover the physics, the regulations, and the practical tests you can do with the phone in your pocket.
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2026-05-04 · 8 min read
How accurate is GPS speed in your phone?
Modern phones report speed within ±0.5 to 2 mph. The accuracy comes from Doppler shift, not just position differences. A practical look at the numbers and the limits.
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2026-05-04 · 7 min read
Speedometer test: how to verify your dashboard accuracy
A 10-minute speedometer test you can do with your phone. Compare your dashboard against GPS at three speeds, calculate the offset, and decide whether to recalibrate.