As long as your cars have the same IP address each time they come in range, this should be possible.
Start out but adding each vehicle to MultiPing (Enter New Target field). You'll have to be pinging them to associate the alert. A 15 second interval is probably pretty good here.
Now, add a new alert with these settings:
Samples to examine: 10
Alert when 10 or more samples are over 9999ms
Event type: Send an email
Notify when alert conditions end (leaves alert state)
For the remainder of the event settings, use your email address / settings as appropriate.
If you want to know when they leave again, an another event to the same alert that Notify: when alert conditions start.
Now, make sure you associate this alert to each of the IP's for the cars. To do this, go to the main screen in MultiPing and right-click each target. From the popup menu, select "Watch this target (Alerts)..." and move the alert you created over to the "Selected Alerts" side. Verify that the target has an [A] indicator on it.
Hopefully, that will do what you're after...