Problem finding speed when using CLLocationManager

I am developing a speedometer application on an iPhone. I used CLLocationManager (GPS) to measure vehicle speed. But I do not get the exact speed. I tried all the filtering methods given in various discussions using stackoverflow. But I do not get any improvements. From time to time, the speed reaches a higher value (> 400 km / h).

I don’t know what went wrong. Please offer me any code to get the exact speed.

Thanks in advance,

I followed the thread stack discussion below

Speed ​​measurement via iPhone SDK

Optimization of CLLocationManager / CoreLocation to quickly get data points on iPhone

CLLocation Speed

Why is my CLLocation speed so inaccurate?

Currently, I have implemented the following code:

//locationManager declaration in ViewDidLoad

locManager=[[CLLocationManager alloc]init];
locManager.delegate =self;
[locManager startUpdatingLocation];           


- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation {

    NSTimeInterval locationAge = abs([newLocation.timestamp timeIntervalSinceNow]);

    if (locationAge > 5.0) return;
    if (newLocation.speed < 0 || newLocation.horizontalAccuracy < 0) return;
    if ((newLocation.horizontalAccuracy < (oldLocation.horizontalAccuracy - 10.0)) || (newLocation.horizontalAccuracy < 50.0)|| (newLocation.horizontalAccuracy <= 150.0))
    {
        if(oldLocation != nil)
        {
            CLLocationDistance distanceChange = [newLocation distanceFromLocation:oldLocation];

            NSTimeInterval sinceLastUpdate = [newLocation.timestamp timeIntervalSinceDate:oldLocation.timestamp];


            double calculatedSpeed;
            calculatedSpeed = (distanceChange / sinceLastUpdate)*3.6;   // to convert the speed into kmph.

        }       
    }

}