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Thread: Which is more accurate? Speedometer or GPS?

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    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.

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    Geoxman KJ6OCL's Avatar
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    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

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    The gps!!!

    BMW speedo's are notoriously about 8-10% optimistic.
    Somers, NY

    Just enjoying the ride.......

  4. #4
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    My speedometer is about 2-3% higher than my gps. I think most brands are "optomistic" in the same way.

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    Registered User chewbacca's Avatar
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    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.
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    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
    '96r1100rt

  7. #7
    Eschewing Obfuscation VTBeemer's Avatar
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    Cool

    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. http://forums.bmwmoa.org/images/icons/icon6.png 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.

    http://www.caranddriver.com/features...ometer-scandal
    Last edited by VTBeemer; 03-24-2013 at 12:31 AM.
    Patrick
    R1200 RT
    K100RS
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  8. #8
    Cam Killer marchyman's Avatar
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    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.

  9. #9
    Registered User dwyandell's Avatar
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    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

  10. #10
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    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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    When I check my speedometer against my onboard computer . The computer usually reads about 2.5 to 3.5 mph slower.

    Checking on a flat road and holding a constant speed and setting the cruise control then getting the immediate reading fro the onboard computer and checking the difference
    Anthony S.
    2012 R1200GS

  12. #12
    Registered User chewbacca's Avatar
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    Quote Originally Posted by 05Train View Post
    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.
    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.
    Old But Not Dead
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  13. #13
    Registered User greenwald's Avatar
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    Quote Originally Posted by DVEINEDER View Post
    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.
    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.

    Law enforcement cycles have their calibrated speedometers certified, so that their pacing is permissible in court for speed enforcement.

    The GPS is the more accurate.
    Last edited by Greenwald; 03-25-2013 at 10:25 PM.
    Kevin Greenwald - MSF RiderCoach # 121656 (BRC,SBRC,IS,IME,SMARTrainer)
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  14. #14
    Debbie's Servant Lee'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.

    The GPS is the more accurate.
    On my work truck the speedometer shows 2 MPH slower than the GPS.
    Lee 2011 K1300S
    MOA # 30878
    Past BMW Bikes, 2003 K1200RS, 1991 K75S, 1987 K75T, 1984 R100RT

  15. #15
    Registered User greenwald's Avatar
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    Quote Originally Posted by Lee View Post
    On my work truck the speedometer shows 2 MPH slower than the GPS.
    This OEM-degradation unique to motorcycles.

    I have cars and SUV's all over the map when it comes to accurate speedometers.
    Kevin Greenwald - MSF RiderCoach # 121656 (BRC,SBRC,IS,IME,SMARTrainer)
    Nationally Certified Law Enforcement Motor Officer (Ret.) / IBA Member #34281
    Motorcycle & High Performance Driving Instructor - ROAD AMERICA Race Track

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