ANSWERS: 3
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I do believe the GPS is accurate. Many cars are inaccurate. If she does not want a ticket...she had better go with the GPS.
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Personally I'd go with the speedometer and if I was unsure of it, I'd use the mile markers on the roadside to calibrate my speedometer (that's what they are for). I use a GPS regularly when hiking and it's funny how I can hike 17 mph sometimes... I know I'm not that fast! lol :)
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A vehicle speedometer should be reasonably close if the tires are the size called out by the manufacture. Portable GPS devices CAN be accurate but cheeper ones may not be. I would suggest using a stopwatch and some highway mile markers to check both. Set cruise at a fixed speed (see table), then adjust speed to reach desired time. now you KNOW how fast you are going. Compare GPS and Vehicle speeds to known speed. Have Fun! Speed/Seconds per Mile 45/80 50/72 55/65.5 60/60 65/55.4 70/51.4 75/48 80/45
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