speedometer not properly calibrated

Thank you sir...

It was and thanks...also thanks to Scott for the data...

The calculations that tell you where you are were muddled up on purpose so the device couldn't be used as a triggering device. Somebody realized that if the device was inaccurate to 30 feet, then the device to be triggered would simply go off 30 feet away, and in the grand scheme of things, this wouldn't make much difference. They changed the calculations a few years ago to give more accurate location data.

Speed data is a bit different. This measures the difference in the location from calculation to the next, and displays the change in location over time as a speed. You are going 60 here, and 30 feet away so the speed accuracy hasn't changed.

GPS works by the receiver finding a minimum of 4 satellites. Withoug getting too precise on the process, the satellites are all synched to the same clock, and they give a pulse at the same time. The receiver simply measures the time it takes for the pulse to get to it, and then calculates where it is on the ground by how long the various signals take to get to it. The speed of the signal is a known condition. For example, radar waves take something like .0037 seconds to go a mile, so if it takes .0074 seconds for the receiver to see the wave, then the target is 1 mile away -- .0037 to go, and .0037 to come back.

GPS is not radar, duh!, but the science is similar -- the pulse takes a known amount of time to travel a known distance, so if we measure the precise time it takes for the signal to get here then we know how far the transmitter is, and with 4 transmitters in space -- there are 24 of them, I think, but we only need 4 to have good data -- then we can measure how far we are from each of them and since we know where they are, we end up with where we are. Speed is merely a calculation based on how long it takes to get from one location to the next.