'Real' Speed not displaying correctly

[Twin4Me]

I went out yesterday on the bike and I seem to have developed an odd problem, the Sat Nav is lagging behind the actual speed I'm travelling. As I move away the sat nav starts displaying the speed something like this:
5mph > 7mph > 12mph > 24mph > 36mph > 40mph > 42 mph

My real speed has been more like this:
5mph > 20mph > 35mph > 40mph

It's lagging behind the speedo both on acceleration and deceleration, once I reach a stable speed it takes about 10-20 seconds to show the 'true' speed.

Pretty annoying and I can't figure out why?

I had a very wet trip back from Scotland a couple of weeks ago, but it was working fine.
I noticed it did an automatic software update when I started it up yesterday.

Anyone else had similar problems?

I can only thing to do a full reset, which is irritating, or contact Garmin for advice.

[jfheath]

Satnavs never show the real speed. They cannot. They can only work out the distance you have travelled in a certain time period - distance / time = speed.

If you accelerate hard, it can take about 5 seconds for it to catch up with your true speed. Of course it is performin a rolling cycle of calculations using the last few 'fixes' (I don't know, but let's say the last 5 seconds), So it will calculate your speed using your current position and your position 5 seconds ago. And if you were going round a corner at the time, the straight line distance will be shorter than the actual distance - so your speed may show slower than it actually is.

(I'm hypothesising - I don't know that the speed is calculated using just two points 5 seconds apart.) But the longer between the readings, the more accurate the speed will be as the positional errors will have lest impact on the calculated speed.

eg at 60mph a bike will travel 29.33 yards in 1 second. Lets call it 30yards to make the calculations easier.
If you take a position fix fix at 0 seconds and another fix at 1 second: The first fix could be 5 yards out in either direction, the second fix could also be 5 yards out in either direction. So the distance travelled could calculated by the satnav to be anywhere between 20 yards and 40 yards.

20 yards in 1 second is 41 mph.
40 yards in 1 second is 82 mph.

That is quite an error.

Do the same calculation over 5 seconds. 150 yards. So with a 5 yard error for the position fixes 5 seconds apart the distance used by the satnav to calculate the speed could be anywhere between 140 yards and 160 yards.
140 yards in 30 in 5 seconds is 57mph
160 yards in 5 seconds is 65 mph

That is much better, but that is an average speed over the distance. By the time it has worked it out and displayed it (almost instantly on the 5th second) , you could be going way faster than that. ie it could be reporting you at 57mph, but you have reached (say) 70mph by the time it displays the speed at the 5th second.

And that is if it has got a good set of good quality satellite fixes - which in some places may not be possible. Mountains, wooded areas, motorways with lots of large lorries, city centres with tall building, tunnels.

The only time you can truly rely on your satnav indicated speed is if youa re travelling in a straight line on an open road with no mountains, trees, traffic obscuring your view of the sky, and travelling at a constant speed. If you stanav maintains the same speed indication, it is probably correct.

On the right hand side of the satnav screen you can display trip data. One of the pieces of data that you can display is GPS Accuracy. Put that onto the screen, it might point to a potential problem. I'm sitting at my desk and have just turned it on and it started off with an accuracy of 70 yards. It has settled down to 9 yards at the moment - but that isn't good enough. Normally when riding I expect to see around 2/3 yards. Sometimes less.

When riding, I don't rely on my satnav speed. My bike is slower than the speedo shows, as most are, but they vary as the tyres wear down and depending on today's air temperature and how long you have been riding. So I use the satnav to calibrate my brain. After riding for a while the tyres will have warmed up. Keep the bike at a steady 60mph on an open road on the satnav, and note what the speedo says. I will then use that for judging my actual speed.

The satnav will use all sorts of tricks to display a more accurate reading. eg many calculations every second and throw away the readings at each extreme.

But my experience is that it doesn't really settle down to a steady reliable speed readout for about 4-5 seconds. That's much faster than you describe, but if you have just turned it on, or if you have moved location while it was turned off, it can take a good few minutes before it gets gets enough satellite signals to obtain an accurate enough positional fix.

The last time I had any real problems like that, I was using the satnav in a Ford car. It had heating elements int he windscreen. Brilliant for defrosting the windows. Not so good for satnavs .

If you have a long tunnel near you, enter it at one speed and then accelerate and keep that speed until you hit daylight again. Watch the satnav speed as you emerge, and watch how long it takes to settle down to a realistic speed.

Ok - that is the long answer. The short answer is that you have put a CB radio next to your satnav, and it is interfering with the radio signals from the satellites !!

[V-Tom]

When you are getting delayed speed reports it usually means that your GPS is not getting a strong reliable signal. It could be because of things blocking some of the satellites and/or just because the satelites are not in a good position to lock onto enough satelites to calculate locations.

Yes when accelerating or slowing down quickly it won't have the most accurate reading but when riding in a straight line at steady speed it will be the most accurate speed that ridera normally have available.


I don't know what "speedo is slow" means but many speedos read higher than the actual speed. Reading 5% to 8% higher than actual is not unusual on V-Stroms.

..Tom

[jfheath]

I don't know what "speedo is slow" means but many speedos read higher than the actual speed.

Yes - mine too. It's a legal requirement that it doesn't read slower. I've corrected that phrase. Thank you.

[Twin4Me]

Thanks for the replies.

The point I was making is that this behaviour is something new, the 'speed shown' was always pretty accurate and would settle within a second or two and read slight lower than my Ducati speedo (as expected).

The problem now is that it's jumping up in random steps and taking 10-20 seconds before it finally approaches a 'matching' speed display with the speedo, and same issue on deceleration. Even riding at a fairly constant speed there is a definite 'lag' which wasn't the case previously.

Seems very odd when these are roads I've ridden frequently, it has never done it before and I have no changed anything on the bike (Especially not a CB radio on a motorbike!)

I sent a message to Garmin who have advised a hard reset so I'll see what that does.

[jfheath]

Just a thought - do you have different maps for the same area ticked in Maps & Navigation / MyMaps ? (eg if you have installed additional maps like OSM) All sorts of funny things happen when you have the same area covered by two navigable maps.

[danham]

This does sound like poor satellite reception, and/or the following: sometimes the almanac (a map downloaded from a satellite which tells the GPS where to look for the birds in the sky) gets corrupted. Not sure how to erase it and start over on an XT, but my guess is a hard reset will do the trick. Then make sure to give the XT a clear view of the sky and leave it for as long as it takes to download the almanac and achieve sat lock.

-dan