GPS speedometer replacement (update)

bsinmich wrote:
I still vote for the GPS speed being more accurate than the auto speedometer. Tire wear alone will account for a couple MPH from new to bald tires. I know that when watching the TV I get the signal 3-4 seconds earlier over the air than by the satellite.

The satellite TV signal travels much further than than the GPS signal. A given TV channel program is fed to an uplink center, either via fiber or for local channels, sometimes over the air. From the local uplink center, the signal travels around 24,000 miles to a satellite orbiting above the equator, where it is then retransmitted down to the satellite TV company's uplink center, another 24,000 miles. The sat service then uplinks the signal again, another 24,000 miles, to its own satellites, where the signal is again retransmitted down 24,000 miles to your own dish and receiver. A total trip of about 96,000 miles. Toss in encrypting and encoding delays, etc., and the time really adds up. On the other hand, GPS signals make a single one way trip from sats orbiting at about 12,600 miles in a variety of orbits that allow usually at least 6 sats to be visible from anywhere on earth. The signal delay is usually around 3-5 microseconds, and the receiver uses multiple sat signals to calculate not only your position, but a time signal correction.