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GPS speed accuracy between brands ??

Discussion in 'Technical Q&A' started by Paul_308, May 5, 2011.

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  1. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
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    #1 Paul_308, May 5, 2011
    I need an accurate highway speed measurement at 60-70mph and my radar gun is inoperative (&fs). So I'm settling on a GPS with speed readout unless a rented device is available. Which gps do you use and what do you feel is it's accuracy at highway speeds?

    I lean toward the big three but am not interested in it's ability to find points of interest and traffic alerts...map function is at the bottom of the desirables list. At the top right now is is accurate speed readout, and somehow it is not printed in their specifications.

    I will be calibrating my 308 speedometer hopefully to 0.1% and need the best reference available. I'm not about to buy a dozen, or search out a dozen state police radar trailers sites. Police radar is often off as they don't calibrate the trailers and the cops just plain lie at times. I've read forums with complaints of a 5mph difference between gps models/brands.
     
  2. Racedrvr

    Racedrvr Formula 3
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    Don
    #2 Racedrvr, May 5, 2011
    Older GPS units with receivers only capable of single digit channels will be less accurate. Newer units with 12 channel receivers and DGPS/WAAS enhancement will be the most accurate. The update rate of the unit will also affect the accuracy of the speed that is displayed while you are changing speeds. Not a big factor in a cruise situation. Therefore I don't think it's a brand thing, but what capability is built into it. Check each unit that you are considering for these factors and you will be able to get the one that meets your needs. Usually the higher end units not only have more accurate speed displays, but also all the other mapping/search functions that it sounds like you don't need.
     
  3. eric355

    eric355 Formula 3
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    #3 eric355, May 5, 2011
    0.1% !!

    1 mm of wear on the tire (considering a 2m circonference wheel) gives already an error of 0.3% on speedo reading....
    Not sure you need to target such an accuracy !
     
  4. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    9,741
    #4 Mitch Alsup, May 5, 2011
    In both my F355 and my 4Runner, my Garmin GPS was within 1 MPH of the spedometer reading. I believe that the GPS was the more accurate of the two readouts.
     
  5. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
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    #5 Paul_308, May 5, 2011
    Thanks Don...just the info I need.

    Eric...this calibrator is capable of a 0.01% correction, so 0.1% is a reasonable target. If the tires wear, I'll change the calibration. A high calibration might come in handy.

    Mitch... 1mph is great, but agreement between the two is the greatest of goals
     
  6. ago car nut

    ago car nut F1 Veteran
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    #6 ago car nut, May 5, 2011
    What about mile marker and stop watch? average out over several miles.


    Ago
     
  7. ramosel

    ramosel Formula 3

    Sep 11, 2004
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    #7 ramosel, May 5, 2011
    Paul,
    There is actually a website dedicated to this....

    http://www.gpsaccuracy.com/

    Rick
     
  8. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    #8 Paul_308, May 5, 2011
    Rick, I was excited when I saw your link but quickly disappointed to find NOTHING...well not exactly nothing. This is the strongest statement I found: "It's hard to say exactly how accurate a portable GPS will be". It's really a marketing site.

    Guess I'll start with anything I can scrounge up or find on sale and check from time to time on borrowed GPS units. The calibrator can easily be changed. I'm just eager to finish this job and post the installation photos and recommend the wiring alteration for a 308. Paid $100 for the speedometer calibrator...now facing twice that for a tool to calibrate the calibrator.

    Incidentally, I found the latest marketing 'buyme' gimic is 'free lifetime maps'. Fine print is “until your product’s useful life expires or we no longer receives map Data from NAVTEQ, whichever is shorter.” Cheap units have $70-$120 map update cost. Higher price units have the lifetime bit.
     
  9. f355spider

    f355spider F1 World Champ
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    May 29, 2001
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    #9 f355spider, May 6, 2011
    Paul,
    There are two different "lifetimes" that are important, lifetime traffic data and lifetime maps. Cheaper units just have lifetime traffic data included. Units with lifetime maps cost a bit more, but I have seen them as low as $140 or so...well worth it when you consider adding it later can easily run $75 to $80 after that fact.

    As Mitch stated, I have found my inexpensive Garmin to be very close to my F355 speedo, and most Toyota and Nissan speedometers. Generally within 1mph.

    The Garmin can be updated every 3 months or so for updated maps, software and accessory data like shopping and restaurants.
     
  10. FerrariDublin

    FerrariDublin F1 Rookie

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    #10 FerrariDublin, May 6, 2011
    I've had a Garmin and a prior model, name of which escapes me now. I reckon both were absolutely accurate as I noticed they under-read the F360 speedo by exactly the same amount. Can't talk about the F355 but it seems that by the time the 360 arrived Ferrari had learnt to be a little more "ahem" enthusiastic (?) with their speedo with differential going on for 10 kph at say 120 kph true speed.
     
  11. SYake

    SYake Karting

    May 15, 2005
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    #11 SYake, May 9, 2011
    Since speed is not directly measured using GPS systems, make sure you have a receiver with multiple channels (or individual receivers) and not that "timeshares" a single receiver for each satellite. Also, the calculated speed is going to be the most accurate on level terrain, since the vertical component of position is the least accurate of GPS receiver calculations. Finally, the receiver with the faster computer clock (calculations per second) will be the more accurate, since the speed you see on the indication is nothing more than change of position divided by update rate.
     
  12. smg2

    smg2 F1 World Champ
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    #12 smg2, May 9, 2011
    another option if you have a an android based phone, there are numerous GPS speedo apps. I've used a few that were surprisingly accurate, the more satellites the better the accuracy. They're free or a couple dollars, some even have the heads up display option.

    I've got to say the number of useful apps for these phones is pretty neat. there's one now that reads OBDII codes via a bluetooth plug-in for your car.

    the future is here....
     

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