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Discussion Starter · #1 ·
Hi, great board.

I've ridden about 1300 km (800 miles) on my new 620, and wonder about the speedo accuracy, which seems to be indicating about 10% high, based on my feeling of the traffic flow compared to when I'm in my c*r.

Any easy way to measure this and if needed, correct it? Is the ECU counting pulses from the speedo, and could this parameter be changed by the dealer?
 
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I was under the same impression when I first got my bike. I found one of those neighborhood speedwatch stations (you know the trailer on the side of the road that displays your speed as you drive past) and took several runs past it at various speeds.... according to the info on the display, my speedo was accurate.

This impression was verified when I got pulled over a few months back by a sheriff's deputy for going 10 over the posted limit (60 in a 50). Fortunately for me, he was a good guy and just gave me a warning after a little chat about good rides (he was a MC patrol officer).
 

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I've also tried checking my speedometer with those speedwatch stations and my reading is pretty close at 35 mph (the typical speedlimit where they're used). I went back to make a run at 55 mph on one of those once and I didn't get a reading at all. 8)
 
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I've got a Garmin GPS on my M620 and noticed that the actual speed is about 10% lower than the speedo says.
The fastest In ever got it to go was 220 Km/h and the GPS said 200 Km/h
 

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A road test done by one of the mags showed only a 2 1/2% high reading on the '03 620. Can't remember which mag, but it was only a few months back. Radar trailer checks seem to show dead-on, so it's close enough for me.

-Don
 

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I made the test once on highway, at the same speed during 100km, take the time and compute my real speed, the difference was under 4%, it's '02 M620S. (and the trip error was also under 4%)

Flavien
 
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I've got a Garmin GPS on my M620 and noticed that the actual speed is about 10% lower than the speedo says.
The fastest In ever got it to go was 220 Km/h and the GPS said 200 Km/h

That's making the awfully big leap of faith that your GPS provides reliable speed info. I'm no GPS expert though, so maybe it is a more reliable measure of speed. All I can say is there's certainly the perception that the ducati speedo is reading fast, but that's more likely due to the fact that your bike is a fine piece of engineering and artistry that rides so nice :)
 

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That's making the awfully big leap of faith that your GPS provides reliable speed info.
Not too big of a leap. Even a crappy GPS is accurate to under a hundred meters, and the time is way more accurate than a $150 piece of plastic deserves to be. To match a 10% accurate speedometer, you only have to take readings over a ride longer than one kilometer. (On short timescales, GPS errors tend to be in the same direction.) To match a 2.5% accurate speedo, you have to ride more than four kilometers. Not a stretch at all. And of course, a good GPS may be accurate to ten meters.
Rene Carlos
 
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Actually, consumer GPS has an accuracy of 5m ...
The one's with higher accuracy are the military ones 8)
I also tested my GPS in comparison to other units.

I went for a ride with my car and took my Garmin eTrex from the bike, another Garmin GPS V from a friend and the GPS thats built into my car. They all showed the same speed (My VW Golf IV has an offset of 5%)

Maybe there is a difference in tacho vs real speed depending on where you got the bike. Mine's the european version ...
 

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Discussion Starter · #11 ·
I will do the math, tachometer x drive ratios x tire circumference to obtain calculated speed. Drive ratios are in manual if I recall
 
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GPS is GPS, the same birds the gov'ment uses are the same your little boxes use. The hardware is even more advanced in the civilian units. One difference is the number of birds you are allowed to use in the civilian models. Military models use 4-7 sats to position and uses a special encrypted channel with greater signal accuracy (PPS). Civilian models use 2-3 birds with the SPS service and they also are cripped with poorer signal detection. Military GPS can be accurate to within a meter or so. Civilian to about 5-10.

All this depends on the number of signals you are able to pick up and calculate. If the military lost some signal (due to jamming, weather, solar flare) and could only use 2-3 birds then their accuracy suffers too.

Comparing a speedo and a GPS is like a blind man to describe a sunset to a deaf mute. The only way to measure accurately is a calibrated speed wheel.
 
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Again, I'm don't know much about GPS (I've just been on one outing where somebody had one, and that was a hike/climb to the top of Mt. Rainier... obviously we weren't measure the units ability to track our speed...

Is GPS an accurate way to gauge land speed?
 
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Is GPS an accurate way to gauge land speed?
accurate? no.

in general? yep.

depends on how inaccurate you can live with it being. Given a good vertical rise, say a steep mountain face the GPS would not note much horizontal motion since you are moving UP. Your physical position does not change much map wise but speed wise it has. GPS just doesnt work very well for this.

Think of it like this. GPS is just 3 points up in space which you triangulate against your position relative to. If you move 10,000 feet closer to these elements your speed could be anything but since GPS only works on a flat plane apparent speed would be zero. Its why GPS based JDAM munitions work so well, they are falling but towards a GPS coordinate. Cruise Missles need their own positioning equipment to guide them to a target in addition to GPS.
 
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So basically, assuming travel along a "flat" surface, GPS would offer accurate speed readings, while travel involving any noticeable elevation gain/loss would provide inaccurate speed readings?
 
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correct, except there is a margin of error in civilian models. Small samples (ie: short distances) would tend to increase the error, large sample (long distances) would tend to decrease the error.

In general if you need to have speed accurate to a few mph or kph within a specific distance GPS isn't designed to do that. But if say you travel on say a hiking trail all day and recorded your distance via way points you can average out your speed based on waypoint times versus distance on a map and come up with an answer.

It is a good tool for a ballpark figure but I wouldnt calibrate my speedo against it. The more accurate method is radar/laser gun but heck they can be off by a few mph and are far more accurate than even military GPS. The only way to gauge the speedo's true accuracy is with a calibrated instrument.

As I understand electronic based speedo's are more accurate at lower speeds than higher, i think due to their digital sampling rates and averaging. I know my mustang is spot on below 40 mph but after 40 drifts low as speed increases so at 65 i'm really doing 62-63. At 100 its about 94-95 (5%!) and only gets worse.
 
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If you don't mind me asking mspgs2, what do you do for a living? I just wonder if Metrology is a hobby or a career.

currently I play with satellites and cable headends.. yawn.
i'm a semi-professional-know-it-all too ;D
 

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My vote is for 10% too high. Basically from the feel of surronding traffic, looking in cagers windows at their speedos, speeding tickets, and the RPM vs tire height vs gear ratio math (do not forget a little fundge factor for tire growth at speed). My samples of the above are enough to say 10% with a
 
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As for GPS, as long as you measured over a long enough distance (not a quick sample) the "built-in" GPS error (assuming it is somewhat constant) will be a non-factor compared to the distance traveled.
with a large enough sample, good enough for guesstimation. I would not want to fight a speeding ticket with it though.
I bet if you traveled on a piece of interstate, 65 miles an hour for 1 hour the GPS would be pretty damn close.
 
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