Speedometer Error

[ProjectM96]

I don't believe GPS' are accurate. If they are, then that means most cars are exactly -2mph off.

My GPS shows 2mph faster than every single car I have used it in. Always 2mph faster whether its in a Honda, Mazda, Nissan, my Porsche, or a Chevy Tahoe.

I get the same result with my fathers GPS and my GF's GPS. Always 2mph faster reading no matter what the car. I tried it with Garmin and Tom Tom and my dad's Android phone.

[southernstar]

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

[pesticidal]

Originally Posted by southernstar

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

This. I sell Farm GPS systems, and even the cheap non-differential GPS are more accurate the the radar systems we used to sell. Typically, within .1 mph