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Actually, especially when averaged over any sort of distance on cruise control, GPS is incredibly accurate. I sail and, while my GPS/chartplotters have WAAS (wide area augmentation system) which improves their accuracy to about 6 feet, nevertheless even without WAAS they are extremely accurate. Consider even a two minute run on cruise at 60 mph: your GPS should be accurate within 24 feet at the worst (12 feet on either side) over 2 statute miles - a margin of error of less than .25% (and likely much less). If your speedometers are always 2 MPH optimisitic (and I have seen variance greater than that on my BMW X3), it is likely because it is programmed into the car for reasons that have already been discussed.
Brad
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