How accurate are GPS speedometers?

[bugo]

I have the Ulysse Speedometer app for my Android phone.  The speedometer in the Taurus I'm driving is broken, and at any speed above 60 MPH or so it starts bouncing like the speedometers in cars in the '60s did.  It also only goes to 85 MPH.  How accurate are these apps?  To be more specific, does anybody have any experience with this app?  I liked the free version but the ads would rotate and sometimes a brightly colored ad would pop up and startle me, so I paid the $2 for the deluxe version.  At times it is obvious that it is off, and sometimes goes to 0 for a few seconds,  but it always corrects itself.  The speedometer is dead on with the speedometer in my Cavalier, but is off in the Taurus (at low speeds when the speedo needle isn't bouncing).  It is also off from the YOUR SPEED IS electric signs near the eastern (southern) tollbooth on the Muskogee Turnpike, but I've decided those signs are inaccurate.

[NE2]

If there's an app that will display positional precision of the GPS receiver, and said precision remains good as you drive, the speed should be accurate. I only have experience with the old standalone GPS receivers.

[realjd]

As long as you're getting GPS reception, the speed coming from a GPS receiver will be much more accurate and precise than a car's speedometer.

[US71]

The speedometer in my van is wonky: sometimes it works, sometimes it doesn't. I have used 2 different GPS speed apps on my phone and both seemed fairly reliable. I get up to whatever speed I want, hit cruise control and I'm set.

[hbelkins]

I haven't had any significant differences between my Saturn's speedometer and my Garmin. (I know my truck speedometer is off by a couple of MPH). I'd say a decent phone GPS app would provide relatively accurate results. Surely to within 5 mph which should be well within the tolerance for speedometer error when it comes to a cop writing speeding tickets.

[wxfree]

Whenever I've driven by a radar speed display, it always shows 2 mph slower than my car's speedometer.  The speedometer reads a little high for some reason, and consistently by 2 mph, whether going 15, 30, or 75 (that radar reading was from a cop, not a sign display).  My GPS speed is also always 2 mph slower than the speedometer shows, suggesting to me that it's quite accurate.  My only experience is with a standalone GPS device.

[andy]

My concern with an app over a dedicated GPS is the potential for other apps/background activities causing momentary blips (like your "0"), but these should be infrequent and should not contribute to persistant errors.

With my device, the MP3 gets interupted with those pesky phone calls.

[triplemultiplex]

As long as you have at least 5 satellites, your GPS will always be more accurate in calculating your speed than your vehicle's speedometer.  The vehicle's speedometer is assuming a certain diameter of tire; something that changes based on air temperature, tire temperature, tire pressure, atmospheric pressure and tread wear.  Not to mention having larger or smaller tires than factory installed.

The GPS is only measuring the position of the antenna of the receiver and with good reception, it's mad accurate; especially at relatively constant, highway speeds.

So unless you're driving through miles of tunnels or box canyons, I would trust the speed of any GPS nav system or smartphone app over the vehicle's speedometer in just about any situation.