The display itself is a gutted marine speedometer fitted with the movement ... [Grant] stuffed an ATTiny85 with a GPS module into the gauge and cooked up a script to convert the GPS velocity ...

Surely the GPS technology should be more accurate than the speedometer? – RP You’re right that GPS is usually more accurate ...

Built for simplicity, elegance, and extreme practicality, Speed is a GPS speedometer that uses the iPad’s location data to tell you how fast you’re traveling. It works with iPads that have GPS ...