How to get car's speed with gps for iphone

- (void) setupLocationManager
{
    self.locationManager = [[[CLLocationManager alloc] init] autorelease];
    if (locationManager.locationServicesEnabled == NO) {
        UIAlertView *servicesDisabledAlert = [[UIAlertView alloc] initWithTitle:@"Location Services Disabled" message:@"You currently have all location services for this device disabled. If you proceed, you will be asked to confirm whether location services should be reenabled." delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];
        [servicesDisabledAlert show];
        [servicesDisabledAlert release];
    }

    locationManager.delegate = self;
    locationManager.distanceFilter = kCLDistanceFilterNone;
    locationManager.desiredAccuracy = kCLLocationAccuracyBest;
    [locationManager startUpdatingLocation];
    self.stateString = NSLocalizedString(@"Updating", @"Updating");
}

- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation {
	
    // test that the horizontal accuracy does not indicate an invalid measurement
 	if (newLocation.horizontalAccuracy < 0) return;

	// test the age of the location measurement to determine if the measurement is cached
    // in most cases you will not want to rely on cached measurements
	NSTimeInterval locationAge = -[newLocation.timestamp timeIntervalSinceNow];
    if (locationAge > 5.0) return;
	
	if ([newLocation speed] < 0.0f) {
[g_Config SetSpeed:0];
[g_Config SetCourseValue];
	}
	else {
		float fSpeed = [newLocation speed];
		[g_Config SetRealSpeed:fSpeed];
		[g_Config SetCourseValue:newLocation.course];
	}
}

- (void)locationManager:(CLLocationManager *)manager didFailWithError:(NSError *)error 
{
	NSLog(@"didFailWithError");
[g_Cofig SetSpeed:0];
[g_Config SetCourseValue:0];
	
    // The location "unknown" error simply means the manager is currently unable to get the location.
    // We can ignore this error for the scenario of getting a single location fix, because we already have a 
    // timeout that will stop the location manager to save power.
    if ([error code] != kCLErrorLocationUnknown) {
        [self stopUpdatingLocation:NSLocalizedString(@"Error", @"Error")];
    }
}

- (void)stopUpdatingLocation:(NSString *)state {
	self.stateString = state;
	NSLog(@"%@", state);
    [locationManager stopUpdatingLocation];
	locationManager.delegate = nil;
}