Leveraging several of a Windows 10 phone's sensor APIs available to the Edge browser, this new PWA (Progressive Web Application) delivers your speed in mph or kph - perhaps to check your car speedo, or perhaps to use instead of it? Yes, if you're navigating then Windows 10 Maps also includes a speedo, but this is in a tiny font - this one is huge and readable from anywhere in the car.
Of course, you can turn this into an 'app' yourself with PAWA or just pin it 'as is' on your Start screen. Or just add it as a 'Favourite'. It's up to you. If you're confused by the different PWA options then check out my feature here.
Here's Speedometer PWA in action:
Shown here is the PWA in action in daytime on my Lumia (so we know it works) and, right, (from a promo shot on an unknown device browser) at night time, which is presumably where the 'AmbientLightSensor API' comes in. Give it a try?