how acurate is the MPH reeading of a car GPS unit?

3 mph hour difference is with in normal limits. Neither the car or the gps is 100%. Car can be affect by tire wear and size of tires. Even gear wear can change it. With the gps there is lag time to the satellite. Only the military gps are pin point accurate

Go with the GPS unit - it is the more accurate device. Car speedometers are manufactured to much less exacting standards and may be ''out'' by as much as 10% at 100 MPH. They are calibrated at the factory to be their most accurate at 30-60 MPH but they still may be 2-3% 'off'.

A GPS unit is only accurate to a certain degree. Basically it has what is called a "calculated" speed. It calculates how fast you are going. The calculation is not always fixed or true to what your speedometer will say because it is a rough estimation. The GPS system was designed to give the driver a similar speed reading, not exactly what is true. The speedometer will of course be more correct because it was built to give you the speed that your car is actually going.

youre gps and Speedometer are probably both inaccurate

speedometers only have to be within 5% of the actual speed

GPS is a fraction of a mph (or kph) off, your speedo could be several mph off, especially if you changed tires or anything like that

yeah my gps has about 3 mph difference but i would take in between both and thats probably ur actaul speed......both arent exactly on!

Mine shows up to 3.5 MPH while parked!

This site is very informative........

http://gpsinformation.net/main/gpsspeed.htm