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GPS moving average speed

I need to find a GPS that will measure moving average to the tenth of a mile per hour at speeds in excess of 100mph. Most consumer units display 3 digits so at over 100mph accuracy is limited to 1 mph. Any suggestions??


  • Tim 1484 Points
    You won't be able to achieve that accuracy with GPS. Even with three digits, at over 100mph the actual error amount is probably far more than 1mph.
  • I have had good luck using a GPS for a speed of 100mph. Trying to achieve a 100mph average over 90 miles from a standing start, I was able to achieve an average speed of 99.9869mph. And this using an Garmin 3600 iQue taped to the dashboard. I figured that there was a newer GPS would allow me even greater accuracy.
  • sussamb 829 Points
    Just because it indicated that speed doesn't mean it was accurate though.
  • My bad, I didn't clarify the situation. I participate in the Silver State Classic Challenge. They use a GPS clock timing system to measure my time over the 90 mile course. My time starts when the light goes green and ends when I break the beam 90 miles later. Based on my elapsed time they calculate my average speed. Average speed 99.9869. I use my GPS to bounce back and forth between 99.9 and 100 mph and then hammer the last mile. I want to run a faster speed but the GPS over 100 mph only show 3 digits allowing for less accuracy. I am looking for a GPS unit that will show overall moving average within .1 mph over 100 mph.
  • Tim 1484 Points
    It doesn't matter much if the speed is above or below 100mph-- a consumer GPS device simply isn't accurate enough for a displayed speed to one tenth of a mile per hour to have much confidence. The fact that it is displayed doesn't mean it is accurate. Even if a GPS were to display mph to one tenth above 100mph I wouldn't trust it to be closer than 1mph accurate.
  • Boyd 2002 Points
    edited December 2014
    Let's try a little quick math… For starters, I'm pretty sure that all consumer GPS devices only take one position fix per second. 100 mph x 5280 feet / 36000 seconds = 146.7 feet travelled in one second.

    Now consider a speed of 100.1 miles per hour: 100.1 x 5280 / 36000 = 146.8 feet in a second.

    Also consider a speed of 101 miles per hour: 101 x 5280 / 3600 = 148.1 feet.

    Consumer GPS devices are only accurate to between 10 to 16 feet under ideal conditions. So the error between two position fixes could be anywhere from 20 to 32 feet. And the GPS uses those positions to calculate speed.

    So for accurate readings of 1/10 mph at 100 mph you would need gps accuracy of about 0.1 feet and for readings that are accurate to 1 mph you would need GPS accuracy of 1.3 feet. No way that you will get this from any consumer grade device. I wouldn't even trust readings to the nearest whole mph.

    I did a review of the Garmin GLO bluetooth GPS receiver for GPSTracklog last year. This unit is somewhat unique in that it takes 10 position fixes per second instead of only one per second. It is not a standalone device however, just a receiver that sends data over bluetooth. You need either a laptop computer or a compatible Android/iOS devices to process that data. And your app must be capable of actually reading 10 fixes per second (older software typically is not).

    Garmin positions this device for aviation use at high speeds where 1 fix per second is a bit slow. However, I did some extensive testing and don't feel that the GLO is any more accurate than other consumer devices in real world use. So the higher sample rate doesn't necessarily translate into better quality data.
  • I am not sure if I am causing confusion or I am confused. (Maybe both.) I want to use the GPS to measure my moving average speed and not my exact speed at any given time. In order for me to cover 90 miles with an average speed of 100 mph I need to complete the course in exactly 54 minutes. I reset the GPS at the starting line and then watch my moving average over the 90 miles. I can and do vary my actual speed over the 90 miles so the exact speed at any given time is not important. If the GPS takes a fix every second, it would have 3240 measurements over the 90 miles and hopefully would give me a accurate moving average over that distance. The problem is that the moving average is displayed in only 3 digits just like the "exact" speed. I was hoping to find a GPS that displays the moving average to the tenth of the mile over 100 mph. If I decide to run a higher speed, say 120 mph, I need to know what my moving average is to the tenth of mph. If my moving average shows 120 mph, it might really be 120.2 mph. Most competitors at this event can come within a second of their target time so being .2 mph over my target average would not allow me to finish in the top five. Does that clarify my problem? BTW, I use an external antenna for the GPS.
  • Tim 1484 Points
    Yes, that clarifies it-- though I'm still not sure consumer grade GPS will be able to achieve the accuracy you are seeking. For example a GPS turned on and not moved will show an accumulation of distance over time as well as a non zero average moving speed. Regardless of accuracy, I don't know of any particular devices which will show that many digits above 100mph, regardless of any actual accuracy.
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