Speedomoter Wrong?
Road Warrior
Joined: Nov 2008
Posts: 1,296
Likes: 3
From: Cheltenham, UK
My understanding has always been that speedo's are intentionally set optimistically by circa 5% and this is a legal requirement so I am intrigued to see some experiences with cars that have matching speeds.
That said, I would not share a blinding faith in what GPS reports either, and suspect that the truth lies somewhere between the 2. In particular, given the way GPS works, it takes little or no account of elevation change so I expect it would be more accurate on straight, flat roads that hilly sections.
I have never noticed a significant difference in Odo reading vs SatNav, but again I would be more inclined to trust the odo
That said, I would not share a blinding faith in what GPS reports either, and suspect that the truth lies somewhere between the 2. In particular, given the way GPS works, it takes little or no account of elevation change so I expect it would be more accurate on straight, flat roads that hilly sections.
I have never noticed a significant difference in Odo reading vs SatNav, but again I would be more inclined to trust the odo
Tourer
Joined: Aug 2009
Posts: 491
Likes: 1
From: Northern VT
Its very common for the GPS to show a difference....its not suppose to be 100% accurate (check the accuracy of the distance shown, Mine usually says something like 10' or 15')...that means its not 100%. I wouldn't expect the MPH to be any different.
Road Master
Joined: Apr 2009
Posts: 1,066
Likes: 9
From: Duluth, GA
The GPS showing a few mph slower is normal...does it on my bike. I used to use another Garmin in my cage and it did the exact same thing....a 2 mph difference slower on the GPS on both the cage and the car.
Had a friend of mine who is an LEO clock me with a radar and the speedo on the bike is dead on. Also ran the distance between mile posts at 60 mph and it took 1 minute on the stopwatch. I am thinking the speedo on the bike is accurate enough.
Had a friend of mine who is an LEO clock me with a radar and the speedo on the bike is dead on. Also ran the distance between mile posts at 60 mph and it took 1 minute on the stopwatch. I am thinking the speedo on the bike is accurate enough.
Outstanding HDF Member
Joined: Jun 2008
Posts: 2,134
Likes: 4
From: Third Coast, South Texas
Sorry, must have missed the 1st time you wrote it,
Novice
Joined: May 2009
Posts: 15
Likes: 0
From: So Cal
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Outstanding HDF Member
Joined: Jul 2007
Posts: 3,341
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From: Woodinville, WA
A few thoughts: Speed on your bike's gauge is based on signals from the vehicle's speed sensor which is located on the transmission (unless you have an older bike - my Evo had a speedometer cable attached to the front wheel). Regardless of cable drive or transmission sensor the assumption is that the tires have a certain circumference and therefore each rotation (be it calculated on drive ratio or simply one wheel rotation) results in a predetermined distance traveled. Realistically this can only be "accurate" when the tire is exactly the right diameter and there are variables that affect the tire's circumference such as make, model, inflation and wear.
GPS, on the other hand, measures the distance traveled in a prescribed time. Measurement of time is relatively accurate so the inaccuracies in GPS speed are, as has already been stated, due to GPS distance inaccuracies. At high speeds the GPS distance accuracy becomes less of an issue so if you want to compare your speedometer against GPS do it at higher speeds, not at 20mph.
GPS, on the other hand, measures the distance traveled in a prescribed time. Measurement of time is relatively accurate so the inaccuracies in GPS speed are, as has already been stated, due to GPS distance inaccuracies. At high speeds the GPS distance accuracy becomes less of an issue so if you want to compare your speedometer against GPS do it at higher speeds, not at 20mph.