How to calculate the car speed on iPhone [closed]

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I want to show a car's speeds on iPhone in real time. What can I use?

Wouldn't using gps to calculate the car's speed take many seconds?

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I have an app called Speedometer and Map. I use Core Location to figure out where the user is, how far they move every second in miles, and then I convert this number to hours, and update it constantly.

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If you implement a class that conforms to the CLLocationManagerDelegate protocol, and set an instance of it as the delegate of a CLLocationManager object, your object will receive -locationManager:didUpdateToLocation:fromLocation: messages after you call -[CLLocationManager startUpdatingLocation]. The CLLocation object you receive via that method has a speed property, which is the speed of the device in meters per second. The frequency of updates can be controlled via CLLocationManager's distanceFilter and desiredAccuracy properties.