Speed warning

[Dustysworld]

My F20 is set to warn me if i go over 50mph. It warns me at 57mph on the speedo . So who's right the navman or the car ?

[mostdom]

Navman! 8)

GPS devices are generally extremly accurate.
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Dom

HERE LIES PND May it rest in peace.
Navigon 7310/iPhone Navigon&Copilot

[Dustysworld]

So my car is out by 7 mph

[mostdom]

If your car is 7 mph faster than your navman. Consider my car is about 5 mph faster which i confirmed when I drove past one of those mobile speed signs that shows you your speed.

Almost undeniable.
_________________
Dom

HERE LIES PND May it rest in peace.
Navigon 7310/iPhone Navigon&Copilot

[cook1e]

I use my Navman N20 in 3 different cars, the car speedo in everyone overreads, the least by about 5MPH when the Navman is reading 70MPH, the most by about 12MPH!

The GPS should be considered accurate if you are travelling at a fairly constant speed, there is a slight lag though when you accelerate or decelerate. I've notice that when coming to s standstill quickly it can still show a small spedd (say 5MPH) for a second or two....

[Dustysworld]

Thinking about it , its good that the car is over calibrated . Making it less likely to get snapped at the cameras, plus the added bonus of the sat nav warning me as well topped up with the fact it warns me of camera locations . I really have no excuse to speed :D

[rodderstrotter]

Driving on an incline can also affect the accuracy of GPS reported speed.

[Gee-Pee]

Rather than thinking about differences in mph, you will probably be more accurate in saying that the speed difference between the GPS and the car speedo is ca 10% - when the GPS displays 50mph, your speedo reads about 55mph and so on.
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Gee-Pee
Lifetime member PGPSW - time rapidly decreasing

[philpugh]

Mechanical car speedos (e.g. ones taking a feed from the drive shaft) are designed to read overspeed - by law (in UK) they have to have an accuracy that prevents them from reading under the correct speed. To do this they are usually designed to read about 7% overspeed.

GPS reported speeds depend on two factors. One is the formula used by the manufacturer to turn changes in location by time into speed - one can assume that these are pretty accurate. BUT the position reported by your GPS assumes you to be on the curved plane (ellipsoid) defined by your GPS datum (usually WGS84 for road nav systems) so your GPS speed is only truly accurate for a vehicle travelling over this mythical surface.

For all intents and purposes travelling along a horizontal straight road will give accurate results. Steep inclines or tight bends will affect the accuracy.
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Phil

[malcolmb1963]

[Q]Mechanical car speedos (e.g. ones taking a feed from the drive shaft) are designed to read overspeed - by law (in UK) they have to have an accuracy that prevents them from reading under the correct speed. To do this they are usually designed to read about 7% overspeed.[/Q]

I lived in the US for a while and found that GPS speed almost exactly matched Speedometer speed. In fact I used to use the GPS speed to set the cruise control at 1mph under the speed limit.

Regarding calculation of speed by GPS, I believe that some systems actually use the doppler effect to produce a speed reading. I had always previously believed that speed was calculated from position change. I'm not an expert (even though I used to work for Garmin) but using doppler probably requires less processing than position change.

[TerryWalsh]

I actually brought my US (actually Mexican) built Ford Focus back with me to the UK. The speedo in this also agrees within 1 mph with my ICN520.

The problem with this is I always seem to be held up by people driving under the limit, particularly in 30 and 40 mph zonesjavascript:emoticon()
Smile

[DRP]

Hi Terry
Unless I have got it wrong, I thought that if a speed zone said 30 I could do any speed up to but not above 30. If 30 mean’s I have to do 30 I think I will have to put an electric motor on my cycle, it’s hard enough trying to peddle in the first instance. 8O
Regards
DRP
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[TerryWalsh]

You are basically correct, however is also also bad manners and I believe an offence to drive too slowly and cause a moving traffic obstruction. My original comment was made with tongue in cheek but the emoticon did not display correctly when I dragged it in to my message (using Firefox).

[DRP]

Hi Terry
I usually pull over and let them pass, when it’s safe to do so. If the speed merchant behind has a greater need to go faster and break the law, that’s up to them I’m on the road, with my job, last year clocked up 69000 miles. The vehicle I drive is classed to do 50 on A roads, 60 on Dual carriageways and 70 on motorways. I have a clean licence and intend to keep it that way, as on anyone day I could loose my licence, then where would I be. That’s why I am here, without my GPS I would be lost.
Regards
DRP
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Sharing information is the key to a better understanding

Mio M400 WEU, N60I, T1, 2x B2, iCN 510 & lots more

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