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Speedo Calibration

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Old Jul 17th, 2020, 17:09   #31
Clan
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Sounds like a guess?

I really do not think contours would play apart for speed calculations, its been fine up til now using the same method for years, do you have any think I can read about it, I can see contours being useful for the drive ahead though, ie serious inclines and declines, especially for towing etc, but I really cannot see it for speed calcs, why add millions more data points into the software when its not really needed seen as speed works just fine.

It just my opinion on the subject and I must point out, I am rarely right.....
I haven't seen a latest model to study in that detail for a while . Yes contours make a noticeable difference to GPS speed readings as to be illegal when you get into into serious hills and mountains .
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Old Jul 17th, 2020, 19:42   #32
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I haven't seen a latest model to study in that detail for a while . Yes contours make a noticeable difference to GPS speed readings as to be illegal when you get into into serious hills and mountains .
I can't understand how though, GPS speed is based on 2 points height means nothing in this case, of course, it can't measure a sheer drop off a cliff much then same way it cannot accurately measure the height of say a drone (I have one) well, it can but it's way out, but that's getting off track, GPS measuring speed horizontally be it on any incline a vehicle can travel is accurate as far as non "real time kinetics" aka rtk GPS as seen in farming etc, contouring is certainly not needed for the general public and any how, the speed is based on GPS plotting, and nothing to do with the map itself, plus, it would need some more serious computing power to, to compute the speed from the contour plots based on GPS speed.
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Old Jul 17th, 2020, 21:38   #33
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Default To return to actual calibration.

Not a Volvo question but throwing this out to the amassed knowledge of this forum.
My son has recently acquired a Vauxhall Insignia VX-Line fitted with 19” wheels and 235/35 VR 19 tyres which will soon require replacement. We wish to fit a higher profile tyre, possibly 235/55. We do not yet know whether the increased size will foul on anything, but, my research indicates that the increased rolling circumference of the 235/55 will seriously affect the speedo. reading causing to read about 15% slow, which is obviously illegal.
Does anyone know whether it is possible to have a speedo. recalibrated to take account of the different tyre size?
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Old Jul 17th, 2020, 21:46   #34
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All things are possible, try talking to Instrument Repair of NottIngham.

They used to calibrate the speedos on police cars and as well as taking the speedo out they also needed to know the size and make of the cars tyres to make the calibration as accurate as possible.
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Old Jul 17th, 2020, 22:05   #35
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Originally Posted by Bashy View Post
Sounds like a guess?

I really do not think contours would play apart for speed calculations, its been fine up til now using the same method for years, do you have any think I can read about it, I can see contours being useful for the drive ahead though, ie serious inclines and declines, especially for towing etc, but I really cannot see it for speed calcs, why add millions more data points into the software when its not really needed seen as speed works just fine.

It just my opinion on the subject and I must point out, I am rarely right.....
Our Merc Actros trucks at work have the contour map programmed into them to operate what Merc call PPC, Predictive Powertrain Control, it means when in cruise control the truck will ease off the gas before a downward hill so when it gains momentum from the hill it still won't exceed the speed limit without braking. The computer also monitors the accelerator, brake and speed, and then calculates the weight of the truck to add into the PPC calculations. Whilst it would seem like a lot of data to hold and manipulate in an onboard computer these days computer capability is massive especially if used in dedicated programmes rather than wasting lots of processing on things like a Windows interface.
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Old Jul 17th, 2020, 23:18   #36
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I can't understand how though, GPS speed is based on 2 points height means nothing in this case, of course, it can't measure a sheer drop off a cliff much then same way it cannot accurately measure the height of say a drone (I have one) well, it can but it's way out, but that's getting off track, GPS measuring speed horizontally be it on any incline a vehicle can travel is accurate as far as non "real time kinetics" aka rtk GPS as seen in farming etc, contouring is certainly not needed for the general public and any how, the speed is based on GPS plotting, and nothing to do with the map itself, plus, it would need some more serious computing power to, to compute the speed from the contour plots based on GPS speed.
well GPS speed only works on a flat road properly think of a triangle the slope being the hill ... a mile along the bottom of the triangle ( the road ) is a mile the distance up the slope of the triangle to the same point is More than a mile isn't it .



an example; a 45 degree slop 44 miles along the flat road at the bottom and if you go up the slope its about 54 miles to cover the same 44 miles in the straight line at the bottom ok a 45 degree slope is a bit exaggerated but hopefully you understand now .

It is similar with curves and bends GPS will calculate it as a series of straight lines .

Its easier to see with a tape measure :-)
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Old Jul 18th, 2020, 05:25   #37
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Quote:
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well GPS speed only works on a flat road properly think of a triangle the slope being the hill ... a mile along the bottom of the triangle ( the road ) is a mile the distance up the slope of the triangle to the same point is More than a mile isn't it .



an example; a 45 degree slop 44 miles along the flat road at the bottom and if you go up the slope its about 54 miles to cover the same 44 miles in the straight line at the bottom ok a 45 degree slope is a bit exaggerated but hopefully you understand now .

It is similar with curves and bends GPS will calculate it as a series of straight lines .

Its easier to see with a tape measure :-)
GPS calculates your speed by constantly updating your "exact" location, measuring the distance traveled since your last recorded coordinate and comparing it to the time taken. Using these two points, it knows your on a slope, however, like I've already said, GPS elevation is not accurate, not unless you have an RTK unit. but, its enough to calculate the speed, the only times it will not be accurate will be when the sky is covered enough to block out more than the minimum number of satellites required but the device you're using. Even if said GPS speed is degraded due to elevation incline/decline, its not going to be by very much, certainly not enough to fret over, we ain't going to be talking 10 mph, more like +/- 2 maybe 4 at most.

Now, if we are talking about newer GPS units built within the last 3 decades will have GPS (Navstar) and GLONASS, the latter being much more accurate, down to around 6ft, these are going to be even more accurate than the GPS counterpart at 5m or if fitted with L5 band (RTK) 30cm but then also they would be fitted with the new Galileo receivers wth accuracy at about 1m down to 1cm encrypted (RTK).

When I 1st used GPS it was on my XDA that needed a BT GPS unit, I used a small tomtom BT device, that used to connect to about 8 sats using the Navistar GPS system, my Phantom 4 Pro drone uses Navstar and GLONASS and connects to about 18 sats with a clear sky, ie no trees or buildings partially blocking the view. With the newer DJI drones able to connect to Navstar, GLONASS and Galileo, the number of sat connections with be many more and the accuracy will be 2nd to none thus, whether you're on a hill or not.


As for the series of straight lines, maybe of concern back when it was Navstar, certainly not any more with the precision of Glonass and now Galileo.
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