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Which is more accurate? Speedometer or GPS?

You're very lucky and have an unusual hexhead.

Maybe not so unusual. Or perhaps it is a GS thing. I've personally checked 3 hexhead GS. Others GS owners I've talked to report the same. < 2 MPH error when compared to a GPS. Also, last week a friend reported his new, 90th anniversary RT is < 2 MPH fast when compared to his GPS.

For Paul: It's EU council directive 75/443/EEC (pdf) that reads:

4.4. The speed indicated must never be less than the true speed. At the speeds specified for the test in 4.3.5 above and between these speeds, there shall be the following relationship between the speed indicated on the dial of the speedometer (V1) and the true speed (V2):
0 ≤ V1 - V2 ≤ V2/10 + 4 km/h
 
I'm going to have a CHP unit do a radar check when I'm in a good position to do it.
And the check of the radar gun is done with this
opplanet-tunning-fork.jpg

As goofy as it sounds........or vibrates :)
 
My experience: The 1987 K75 seemed to be absolutely accurate with both odometer and speedometer. (Checked the odometer against US mileage signs over many miles. No GPS, but somehow mounted a wrist watch so I could see the second hand as I passed the mileage signs on flat straight roads and concentrated on maintaining 60 mph. Sure enough; each mile took 60 seconds.) The current K100RS seems to have just as accurate odometer, but the speedo is quite high.

Any explanation? I would think the two should be perfectly correlated.
 
My CLC speedometer is almost 8mph over true speed at 80mph. My GPS is 1mph over true speed and both units were verified by a friendly Peace Officer on the North side of Jacksonville, FL. The road had no traffic and the Officer verified my speed with his radar unit in a 65mph speed zone. My HD Softtail is only 1 1/2mph off at 60 mph indicated.

RIDE SAFE
 
I'd guess that the cop's radar was reading 1 mph low rather than the GPS reading high. There are too many variables with police speed radar.
 
I'd guess that the cop's radar was reading 1 mph low rather than the GPS reading high. There are too many variables with police speed radar.
I'm a certified Geographic Information Systems Professional. Geographic accuracy is my business and I use mapping grade GPS and work with surveyors using survey grade GPS. Been to NavTech classes on GPS accuracy. Ran several GPS base stations for differential GPS before new HARN system differential nets were available, took classes on real time kinematic GPS. Bottom line I know what these Garmin units are capable of and far too many people believe TV cop shows about the accuracy of GPS. Last but not least, GPS accuracy varies over time and place. That is, it may be more accurate in Memphis at 12 noon than in Denver at 2 am. This leads to inaccuracies in both position and speed measurements. Having GPS a couple mph off is not unusual. Also, a radar gun has issues too.
 
2003 R1150RS - speedo is ~ 1mph faster than Garmin Nuvi 1450 across the speedo range (at least to 80mph - above that I think it's a moot point).
 
And the check of the radar gun is done with this
opplanet-tunning-fork.jpg

As goofy as it sounds........or vibrates :)

Actually we have 2 tuning forks to use at the start of a shift. One is manufactured to emit a frequency that the radar transceiver (in 'stationary' mode) will interpret as 35 MPH and one that will resonate a frequency equal to 65 MPH. We strike the forks against a hard but dull object (usually the steering wheel or a nearby supervisor). Reason for that is to not chip pieces out of the metal by hitting them against each other (metal-on-metal).

Once enough metal is chipped away, the lower mass will alter the resonance, and hence the intended 'speed.'

Finally, we get both tuning forks resonating at the same time and set the radar to the 'mobile' mode, where it can interpret multiple signals and calculate the difference. It needs to indicate a net of 30 MPH before calibration is considered confirmed and the LEO begins his/her tour of duty.

The multiple signals allow the radar to discount the squad car's velocity and accurately report an offender's speed when the LEO is operating radar while driving down the road in either direction.

Probably more than you wanted to know, but not privileged information - everyone entitled to such info.

Have a nice day. :wave
 
ALL motorcycle manufacturers degrade the accuracy of their speedometers by 3-7%, making them read faster than your actual groundspeed. This avoids what could have been a deluge of accident litagation.

BMW's service bulletin on this explained it differently and indicated it's an EU regulation that it be this way.

What there is NO regulation for is whether two different manufacturer or even same manufacturer/different model tires are in fact exactly the same size when actually marked the same. Tires change size when they wear, too. This is compensated for by making certain the speedo will never read low.
 
Losing .250 inches of tread would offset the speedometer about 1 mph. Most bikes I've heard about are 3-5 mph off. they're padding the speedo to compensate for all the other factors
 
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