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Thread: Which is more accurate? Speedometer or GPS?

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  1. #1

    Which is more accurate? Speedometer or GPS?

    There is a difference of approx. 5mpg between my Zumo 665 and the speedometer. The GPS has me going slower than the speedometer shows. So which do you think is more accurate? I assume it is the GPS, but I don't wish to bet a speeding ticket on a hunch.

  2. #2
    Geoxman KJ6OCL's Avatar
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    Sep 2011
    Trinity County, Calif.
    I have estimated that my speedometer on my "C", reads somewhere between 3-5 mph faster than actual speed. I'm going to have a CHP unit do a radar check when I'm in a good position to do it.

    Lkarl KJ6OCL / 2000, R1200C

  3. #3
    Fortis Fortuna Adiuvat Omega Man's Avatar
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    Jan 2010
    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
    "You can do good or you can do well. Sooner or later they make you choose." MI5
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  4. #4
    Kbiker BCKRider's Avatar
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    Jan 2009
    B.C. Canada
    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.
    1992 K100RS

  5. #5
    Rpbump USN RET CPO Rpbump's Avatar
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    Jan 2008
    Jacksonville, FL
    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.

    Cave Contents: 1980 R100RT/Ural Sidecar, 2004 R1200CLC, 2006 HD FSXTI
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  6. #6
    Registered User
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    Apr 2012
    Gilbert, AZ
    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.
    2004 R1150RT
    BMW MOA 181289

  7. #7
    Registered User greenwald's Avatar
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    Dec 2006
    Sheboygan, WI


    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 - MSF Lead RiderCoach # 121656 (BRC,SBRC,IS,IME,SMARTrainer)
    Nationally Certified Law Enforcement Motor Officer (Ret.) / IBA Member #34281
    Motorcycle/High Performance/Teen/Winter/ATV Driving Instructor - ROAD AMERICA Track

  8. #8
    Registered User
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    Aug 2005
    Charlotte, NC

    The gps!!!

    BMW speedo's are notoriously about 8-10% optimistic.
    Enjoying the ride, but always on the alert for a rally.......

  9. #9
    Rally Rat
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    Nov 2005
    Lancaster County, PA
    My speedometer is about 2-3% higher than my gps. I think most brands are "optomistic" in the same way.

  10. #10
    GPS is thought to be super accurate but it is not as accurate as most believe. The most accurate method I have used was a heads up display unit I once bought that measured time and tire rotation. You could calibrate to be dead on. GPS is NOT accurate in turns and at high speeds. It depends on sampling rates, PDOP at the collection time, and visibility of satellites. Also remember, even with the best conditions and using WAAS you only have maybe plus or minus 10ft accuracy on the position of each sample. The speedo can be re= calibrated to be very accurate using a box that I totally fail to remember its name. Funny, on Triumphs the speed is 5-10% off but the odometer is dead on. When you recalibrate the speed, the odometer is off 5-10%. Most people believe the manufacturers want you to believe you are going faster than you really are for liability reasons.
    Old But Not Dead
    Semper Fi

  11. #11
    Registered User
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    Jan 2012
    New Rochelle, New York

    Which is more accurate? Speedometer or GPS?

    Just went for my first long pre-Spring ride the other day and made the same observation regarding my GPS's speed vs. the bike speedo.
    Gps was a constant 3MPH faster from about 40 to 75. Around 80+ (hey, ya gotta get that gas stabilizer out of there somehow) edging the gap a bit wider. Since the band of difference is consistent of course there is some speedo error but I'm chalking it up to the difference between whatever came as a stock tire on my 16 year old bike from the 90's to my 2013 pr3's that have barely 700 miles on them. Diameter and basic structure of tire. Am I on the right track? Doesn't bother me that I may be going ever so slightly faster than the bike says. The clock runs a bit faster too.

    mike wex

  12. #12
    Eschewing Obfuscation VTBeemer's Avatar
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    Nov 2009


    Car and Driver article referenced below is a bit dated from 2002, but brings up some bias in speedometer reporting and issues about tire over/under inflation and even how tire profile type can impact it. When I was growing up there was always some rumor about set higher so the car manufacturer would not be liable for speeding tickets on under stated MPH readings. Guessing Beemer bikes are no different. Here's a couple of excerpts out of the article:

    * When traveling at a true 70 mph, as indicated by our highly precise Datron optical fifth-wheel equipment, the average speedometer (based on more than 200 road-tested vehicles) reads 71.37 mph. Wait, wait! Before you roll your eyes and turn the page, let us dig just a bit deeper and reveal some dirt.
    * Sorted by price, luxury cars are the least accurate, and cars costing less than $20,000 are the most accurate. By category, sports cars indicate higher speeds than sedans or trucks. Cars built in Europe exaggerate more than Japanese cars, which in turn fib more than North American ones. And by manufacturer, GM's domestic products are the most accurate, and BMW's are the least accurate by far. One other trend: Only 13 of our 200 test speedos registered below true 70 mph, and only three of those were below 69 mph, while 90 vehicles indicated higher than 71 mph. Are our cars trying to keep us out of traffic court?
    * In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial.
    Last edited by VTBeemer; 03-24-2013 at 12:31 AM.
    R1200 RT

  13. #13
    Quote Originally Posted by sdpc2 View Post
    BMW speedo's are notoriously about 8-10% optimistic.
    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.

  14. #14
    Registered User dwyandell's Avatar
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    May 2011
    People's Republic of Vermont
    An inexpensive Sigma bicycle speedometer (e.g. 1009) is more accurate than your OEM BMW speedo and tends to agree with the GPS (if you've measured your tire size accurately). Other brands are apparently not reliable at motorcycle speeds .. . but there's a ton of info on the web about using Sigma's on motorcycles.
    It'll also give you a couple of extra trip odometers, record max speed, average speed, elapsed time, display the correct time, etc., etc, for under $30, not to mention the fact that you'll have an accurate working speedometer during those intervals when your OEM BMW speedometer is off at Palo Alto being rebuilt AGAIN and your offspring have swiped your GPS. And if you change tire profiles, it is easy to recalibrate. It takes about 10 minutes to mount and worth every penny IMO.
    Dave in Vermont
    '84 R80ST
    '81 R100 hack

  15. #15
    Rally Rat
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    Apr 2012
    The GPS is far more accurate than the speedometer. On a level road, it's as accurate as anyone would need it to be. My RT had a small difference between indicated and actual, my GS/A is 6mph optimistic at 70mph. There's a nifty bit of software that can correct this on other bikes, I'm surprised the GS-911 can't do it for ours.

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