HAH! For once, a tech question on the forums that I can answer with authority!!
(I am a Space Systems Analyst...yeppers a real live rocket scientist
In almost every case, with a modern GPS unit (>2 years old), the GPS will beat out the odometer (or speedometer). This especially holds true with a older vehicle, one with worn tires, or a vehicle that uses a mechanical odometer/speedometer!
So how doe it work? Glad you asked (as he takes a pointer out of his pocket protector
No matter where you are, at least four GPS satellites are able to be acquired by your GPS unit at any given time. Each satellite transmits data packets to your GPS recever (technically we refer to them as PNT packets – Position, Timing & Navigation).
Transmission and reception are constant (signals travel at light speed), thereby allowing the GPS receiver to calculate how far away each satellite is based on how long it took for the PNT packet to arrive (there are cross-comparisons but that is TOO technical!
Now that the GPS recieverin your vehicle has this information, it takes the data from the 3 strongest satellite signals and geolocates your position using a process called trilateration.
Here is a quick and dirty explanation of trilateration: Imagine you are standing somewhere on Earth with three satellites in the sky above you. Each satellite has a certain coverage area. If you happen know how far away you are from satellite A, then you know you must be located somewhere in Satellite A’s coverage area. If you do the same for satellites B and C, you can work out your location by seeing where the three coverage areas intersect (think 3-way VENN diagram here)
This is pretty much what your GPS receiver does, (although it does it in three dimensions, using overlapping spheres rather than circles).
So, howthehelldoes a GPS know what time it is (including auto adjusting for time zones?) GPS satellites have on board atomic clocks to keep accurate time (the T or timing part of the PNT packet
General and Special Relativity predict that differences will appear between these clocks and an identical clock on Earth:
General Relativity predicts that time will appear to run slower under stronger gravitational pull – the clocks on board the satellites will therefore seem to run faster than a clock on Earth.
Special Relativity predicts that because the satellites’ clocks are moving relative to a clock on Earth, they will appear to run slower.
The GPS network makes allowances for these effects – which is SOLID proof that all that Relativity mumbo-jumbo you learned in High School or College physics has a real world impact!.
I stole the graphic below from a GPS briefing I gave a while back – hope this helps you visualize what I wrote above!
(Hands out aspirin to those who have headaches by now