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Thread: Which is more accurate? Speedometer or GPS?

  1. #16
    A bozo on the bus deilenberger's Avatar
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    Quote Originally Posted by marchyman View Post
    Don't own a modern beemer, do you?

    My hexhead speedo reads about 1 1/5 MPH faster than a GPS at all speeds I travel. According to reports I've read most hexheads read 1-2 MPH fast.
    You're very lucky and have an unusual hexhead.

    My '07 R12R reads 5MPH fast at 60MPH, and close to 8MPH fast at 75MPH. BMW once wrote up their allowable error margin, and it's a formula that includes a simple MPH error and a % of MPH error. It is always a positive error (reads higher then real speed.) The claim made at the time was they were trying to avoid ever having a speedo that read slow. My Porsche speedo agrees within 1MPH with my GPS... so it can be done and the Germans know how to do it.

    I've always relied on my GPS for accurate speed on the bike. Anytime I go past one of the radar speed warning signs the GPS has been in perfect agreement with the sign, the speedo exhibits it's usual error. This is true of many other reports from other riders when we've discussed this.

    Know how in a BMW car passengers will say "It doesn't feel like it's going 100mph.."? That's because it's actually doing somewhere around 90 (currently own 1 BMW car, and have owned many of them.)
    Don Eilenberger http://www.eilenberger.net
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  2. #17
    Mind is not for rent
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    Quote Originally Posted by CHEWBACCA View Post
    A GPS is accurate on flat, straight pavement, BUT, it drops off significantly in curves and where there are large elevation changes. It also depends on time of day and date. Why? Because the accuracy of any GPS reading depends on the configuration of the satellites. Those configurations change moment by moment. It is basic solid geometry. Before surveyors go out to take GPS readings they check for the best configuration times. I believe that nifty piece of software you speak of is one of two I am familiar with: TuneBoy or TuneECU. They program the ECU directly which GS911 does not do. There is a box called Yellow Box that calibrates MC speedo's and another called something like Speedo Calibrator.
    Actually, I was referring to the TTS Mastertune. Wonderful tool, wish it wasn't Harley-only.

    "Significantly" is somewhat misleading. Your GPS speed will not read 10mph off when you go up a hill or go through a corner. We're not talking about surveying where 1/100th of an inch can make a difference, we're talking about vehicle speed and odometer corrections. You can feel very confident that if your GPS says you're going 70 on a level highway, you are in fact going 70.

  3. #18
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    Which is more accurate? Speedometer or GPS?

    So I guess I'm in a minority that my bike shows I'm actually going slower (always 3mph slower) than the GPS indicates at almost every speed.
    I still believe that while there is some minor inaccuracy on the speedo the difference in stock vs. new tire pattern/diameter/style has a lot to do with it. It's too consistent.

    mike wex
    '96r1100rt

  4. #19
    Registered User richardak's Avatar
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    The cynic in me suggests that having the speedometer (and odometer) read higher than actual is so your warranty runs out a little bit faster than it should. On my truck, I can adjust tire diameter in 1/20" on the computer so I was able to match the GPS within a couple of tenths of a mile on a 200 mile trip. Now the speedometer matches the GPS and most of those electronic signs that seem to be popular in construction and school zones. i wish it was that easy on every other vehicle.
    1983 R100RT hacked w/Cozy Rocket My blog
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  5. #20
    Benchwrenching PGlaves's Avatar
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    It is my understanding that German regulations (I am not a lawyer, and not German) allow a speedometer to read as much as 10% high, but never more than 0% low. So within a range of error for manufacturing tolerances they are designed to read somewhat (say 5% high) which is the happy middle of the allowable range. Speedo's on authority bikes have a different part number than the civilian version too.
    Paul Glaves - "Big Bend", Texas U.S.A
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  6. #21
    Cam Killer marchyman's Avatar
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    Quote Originally Posted by deilenberger View Post
    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

  7. #22
    Outlander Omega Man's Avatar
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    Quote Originally Posted by KJ6OCL View Post
    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
    As goofy as it sounds........or vibrates
    "Well they say.. time loves a hero but only time will tell.. If he's real, he's a legend from heaven If he ain't he was sent here from hell" Lowell George
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  8. #23
    Kbiker BCKRider's Avatar
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    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.
    Doug
    1992 K100RS

  9. #24
    Rpbump USN RET CPO Rpbump's Avatar
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    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
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  10. #25
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    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.
    1983 R100RS (Sold)
    2004 R1150RT
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  11. #26
    Registered User chewbacca's Avatar
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    Quote Originally Posted by MARKAZ View Post
    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.
    Old But Not Dead
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  12. #27
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    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).
    JWMcDonald
    2003 R1150RS (current) / 1978 R80/7 (prior)
    1966 R69S (prior) / 1972 R75/5 (prior)
    Windsor, California

  13. #28
    Registered User greenwald's Avatar
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    Quote Originally Posted by Omega Man View Post
    And the check of the radar gun is done with this
    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.
    Kevin Greenwald - Touring Tips Editor
    Nationally Certified Law Enforcement Motor Officer (Ret.) / IBA Member #34281
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  14. #29
    Registered User lkchris's Avatar
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    Quote Originally Posted by Greenwald View Post
    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.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

  15. #30
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    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
    Anthony S.
    2012 R1200GS

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