Speedometer best way to determine vehicle speed

Question: I'm writing to you to ask a question regarding whether or not you might know where the signal is that I can only assume is the method to tell current "smarter" cars what the speed limit is on the road or highway they are currently driving. I wonder how accurate they really are.
Answer: Never rely solely on the displayed GPS speed reading. Your vehicle’s speedometer is the best way to determine how fast you are traveling.
A GPS is a positional speedometer. It will show your speed based on the average distance traveled in the last few seconds. The speed readings could be inaccurate if there is a temporary loss of signal.
Always be alert and aware of the posted speed limit signs while driving. Remember that the speed limits start and end at the location of the sign.
Be sure to program your destination on your navigation system prior to driving and remember that the new hands-free law is currently being enforced in Minnesota.
The new law allows a driver to use their cell phone to make calls, text, listen to music or podcasts and get directions, but only by voice commands or single touch activation without holding the phone.
You can avoid a ticket — and a crash — if you simply buckle up, drive at safe speeds, pay attention and always drive sober.  Help us drive Minnesota Toward Zero Deaths.
If you have any questions concerning traffic related laws or issues in Minnesota send your questions to Sgt. Troy Christianson – Minnesota State Patrol at 2900 48th Street NW, Rochester MN 55901-5848.  (Or reach him at, Troy.Christianson@state.mn.us)