garygid said:
Any car's indicated speed (designed to DISPLAY high) needs to be checked against a simple handheld GPS's speed (almost always quite accurate).
As someone who has owned 3 different handheld GPS units and one dash-mount unit, I can tell you they are terrible at top speed measurement. I have driven on the freeway at a steady 60 mph for a measured 3 miles to test the speedometer and GPS accuracy. I finished the measured 3 miles in exactly 3 minutes, confirming the 60 MPH speed. The car speedometer never wavered more than 1 dot on the gauge. The GPS unit (a high-end Garmin) showed a top speed of over 80 and a low speed of 40-something. I've tested them in planes, too. While cruising at altitude in a commercial airliner they have shown a speed of over 900 MPH. Not possible. They are simply unreliable for measuring top speed or short distance. This is because they take a reading every second or so and can be off by as much as 20 or 30 feet each time. Sometimes they miss a reading due to interference (trees, mountains, buildings) or get a poor reading with only three satellites, increasing inaccuracy to as much as 50 or even 100 feet. If you are traveling 60 MPH you are going 88 feet every second. So if you head northbound at 88 fps and the GPS reads your position further N by 20 feet than it really is then 1 second later reads it as further S than it really is by 20 feet, it has subtracted 40 feet from the actual distance you traveled, thinking you are going barely over half your true speed. Similarly if the readings are inaccurate the opposite direction you can be 50% faster. This problem is lessened if the readings are less frequent, but then you get a different inaccuracy if you are on a curved road or trail since the distances the GPS measures are straight line. So the maximum and minimum speeds on GPS units are often very inaccurate. If you only look at average speed over a considerable distance on a straight level road, then they are usually quite accurate, but not as accurate as your car's speedometer. If you really want to check the speedometer's accuracy you need to use a police radar, or better yet, lidar. Those electronic signs that tell you your speed are also very accurate if you are the only car in range.
