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How accurate are GPS speedos?


Huzband

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Zip-tied the 276C to the top clamp(bracket's on the way,but I couldn't wait)to play around. I know BMW speedos are notoriously optomistic,but it made me think about the GPS. 30 on the bike is only 26 on the receiver,& 80 is 72. So now I'm wondering if I can count on the 276 reading,& still stay out of trouble.

What do you think?

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Joe Frickin' Friday
Zip-tied the 276C to the top clamp(bracket's on the way,but I couldn't wait)to play around. I know BMW speedos are notoriously optomistic,but it made me think about the GPS. 30 on the bike is only 26 on the receiver,& 80 is 72. So now I'm wondering if I can count on the 276 reading,& still stay out of trouble.

What do you think?

 

GPS speed readings in general are accurate to something like 0.1 mph. Garmin gives the specific features for each of their models on their website, including the 276c, which they declare accurate to within 0.05 meters per second (0.112 mph).

 

More on GPS in general here.

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Huzband, you're getting about the same readings I got on my 02 RT with a 276c. It is now to the point where I don't even look at the bike speedometer and implicitly trust the GPS. My new bike is off between GPS and speedometer by about 1-2 mph, but the RT was around 5mph, until as you noticed, you get up above 70mph, and then it gets real fuzzy.

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One of our UK LEO members has checked out his GPS against his official calibrated speedometer and has found it to be a good match.

 

Andy

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Effervescent

I have a Garmin GPS V. It's dead-on-balls accurate with the speedos on my RX-7 and M3, at any speed. My other vehicles (including my 1100S) vary between 3 and 9 MPH depending on speed, to the Garmin.

 

I trust the Garmin and the Garmin only. thumbsup.gif

 

-Eff

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ShovelStrokeEd

Provided you are riding on a straight and level road with a good enough view of the sky to give you 4 or 5 strong satellite signals, your GPS should be very accurate. I have done a lot of calculations in regards gearing, tire diameter, wheel size, aspect ratio and the like. I also have three bikes with sligtly different gearing but the same tires on them. Calibrated one bike against the GPS by correcting the aspect ratio of the tire so that tachometer RPM and mph in each gear actually corresponded to the GPS readings. I could then plug in the gearing from the other bikes and get exact relationships between engine RPM and mph. BMW speedo on my 1100S off by 8.2%, Honda speedo on my Blackbird, off by 8.6%, Honda speedo on my VFR, off by 8.1%.

 

This suggests to me that when the various manufacturers do their speedometer gearing or pulse per rev calculations, they are working with the nominal dimensions of the tires and not actual dimensions as measured by rolling the laden tire through one revolution. The aspect ratio being the culprit there with a nominal 180-55 tire actualy coming out to more like 180-52.5.

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I appreciate everyones input but,good grief Shovel,reading your response is like reading a Kevin Cameron article! tongue.gif

 

Seriously, very informative, & thank you. thumbsup.gif

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ShovelStrokeEd

Answers to technical questions is part of what I do for a living and I have found the more information I can provide, letting the reader pick out what he needs, the less I have to repeat myself.

 

BTW, thanks for the comparison to Kevin Cameron, I think he is one of the great explainers.

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Lone_RT_rider

Ed,

 

I think that that level ground issue is bigger than we give it credit for. I can hold my speed at an indicated 70 mph while going up and down hills and watch the GPS vary +/- 5 mph. I haven't trigged out the calculations but from an intuitive standpoint it make sense.

 

*NOTE* I fully expect the more bored amongst us to come up with the calculations .....

 

Shawn

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Joe Frickin' Friday
I think that that level ground issue is bigger than we give it credit for. I can hold my speed at an indicated 70 mph while going up and down hills and watch the GPS vary +/- 5 mph. I haven't trigged out the calculations but from an intuitive standpoint it make sense.

 

*NOTE* I fully expect the more bored amongst us to come up with the calculations .....

 

<yawn>

 

if this is true, then your GPS only displays the horizontal component of your total velocity; worst case would be on a steep mountain road, 13% grade (13 feet vertical for every 100 feet horizontal), where your true road speed is 0.8% higher than the speed on the GPS display.

 

If your GPS speed is jumping around while your true vehicle speed is steady, then you may be encountering multipath errors.

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Lone_RT_rider
I think that that level ground issue is bigger than we give it credit for. I can hold my speed at an indicated 70 mph while going up and down hills and watch the GPS vary +/- 5 mph. I haven't trigged out the calculations but from an intuitive standpoint it make sense.

 

*NOTE* I fully expect the more bored amongst us to come up with the calculations .....

 

<yawn>

 

if this is true, then your GPS only displays the horizontal component of your total velocity; worst case would be on a steep mountain road, 13% grade (13 feet vertical for every 100 feet horizontal), where your true road speed is 0.8% higher than the speed on the GPS display.

 

If your GPS speed is jumping around while your true vehicle speed is steady, then you may be encountering multipath errors.

 

I told you there were people more bored than I out there... grin.gif

 

Shawn

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... the more information I can provide, letting the reader pick out what he needs, the less I have to repeat myself.

"Bullshit baffles brains," as we used to say. grin.gif

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I found the same on my 99 RT, to be 5 mph off or more at higher speeds. I used that as my speedo but the damn heat here in AZ, 120 on Tuesday at my house in the West Valley, kills the plastic mounting bracket on my shelf. So I am going by the speedo but just subtracting 5 from my current speedo speed.

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I have to wonder about these GPS's my max speed is shown as 205.1 miles per hour. I have been using it for the last two years there are a lot of discrepancy's - most minor though.

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I have a 176 [antique!!], and it depends on the number of sats the unit can view - which relates to where you are physically to the angle of view. Same for elevation readings. The services still get acc that is far and above what we can get. I use mine in my D-90 off road thumbsup.gif and find that the best it can give me in the desert is about 8'. Of course I am not sure if I really need more definition [except for speed ??] grin.gif.

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