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GPS versus Speedo Accuracy

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HawkGTRider:
I have a series of curves on one of my routes to and from home, and I'm guilty, at times, of seeing how quickly I can go through them. Like Tosh, I am focused on looking through the curve and only observe my speed with a quick glance as I exit the maneuver.

OldButNotDead:
Tosh, Geoffrey,
I'm right with you on keeping eyes on the road.  My primary exit into the great beyond is a four or five mile stretch of road that has several "clear sight line" 20 mph turns.  That's where I do my experimenting.  I know the turns and can almost run them blind (blatant lie lol) .  Just wanted to see what speed I was actually taking them and that is where I noticed the difference between speedo and GPS, not to mention just by feel I knew I was not dropping speed in the turn.
Rod

stevegrab:

--- Quote from: DirtFlier on July 31, 2017, 07:47:13 am ---I'm almost always looking through the curve and not down at my GPS.  :-)

--- End quote ---
This and I probably pay attention to my speed as much as anybody so that I know if and by how much I'm breaking the limits.
I have noticed on some mountain roads my GPS showing my bike dancing off road somewhere, because it is not accurately reflecting my location. So I presume similar things impact the speed readout, which is still far more accurate than any bike speedo I've had (newewst bike is over 10 years old).

NinjaBob:
This anecdote may have no relevance to this topic but... I run with a GPS enabled trainer on my wrist, Garmin Forerunner 305.
It seems to be as accurate as my Nuvi and Zumo. But I noticed an elevation discrepancy of about 40 ft depending on whether I have my wrist at my side or have it raised to eye level. I figure the antenna is more accurate at a certain angle. So besides the reasons mentioned above for the speed differences maybe the lean angle of the bike in a turn affects the accuracy.

JimRRides:
Corollary to GPS accuracy for speed

In the last few years I have observed officers and prosecutors referring to the GPS recorded speed on the video from the patrol vehicles as proof of the defendant's speed. Rarely does the defendant challenge the assumption that the GPS speed is an accurate measure. It is NOT an accurate measure of the suspect vehicle nor the patrol vehicle, although it may be referred to as an approximation. This source of speed measurement has never (to my knowledge) been vetted by the courts in a manner similar to speed radar. If you or an acquaintance face an accusation based on this measure,  challenge it. (Note that some video systems record the readout from the actual speed radar unit and thus is a different issue. Our traffic units had this capability but not our routine patrol units.)

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