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[Lampynoiseboy]

My Tomtom always tells me I'm going 3mph slower than my speedo says. Seeing as I tend to do Gatsos at something approaching the alleged limit, it's a small concern.

I would assume the car would be more accurate, but if my speedo's out....

Thoughts?

[Entwood]

All speedos are set to "overread" as it is illegal for them to "underread", most overread by between 3 and 5 mph .. as you have seen.

Satnav speed CAN be very accurate .. but unless you understand it, can also be very inaccurate !!!

Satnav gets the speed by calculating the distance between two points, in 3 dimensional space, and how long you take to cover that distance.

If you travel at a constant speed along a straight, flat, road .. the satnav will be accurate.

If the road has bends, or hills, the satnav will be inaccurate as you will actually travel further than it "thinks".

HTH 

[Lampynoiseboy]

All speedos are set to "overread" as it is illegal for them to "underread", most overread by between 3 and 5 mph .. as you have seen.

Satnav speed CAN be very accurate .. but unless you understand it, can also be very inaccurate !!!

Satnav gets the speed by calculating the distance between two points, in 3 dimensional space, and how long you take to cover that distance.

If you travel at a constant speed along a straight, flat, road .. the satnav will be accurate.

If the road has bends, or hills, the satnav will be inaccurate as you will actually travel further than it "thinks".

HTH 

I had heard this, but thought it was an urban myth, does make sense tho- erring on the side of caution as it were.

In the words of my very chavvy 18-year-old cousin "Big up urself"

 

PS, not a phrase I use.... my car has a boot 

[TheBoy]

The speedo is factory set to be around 2mph over, though component tolerance plus age does make this vary. Also, the tyre wear will have a small effect.

For speed cameras, I'd go by the speedo.


Note, whilst a proper GPS speed monitor can be mega accurate, satnav devices are not, due to the way they work. I certainly would trust the speed reading on a satnav, even on a dead straight road.

[Andy B]

Humberside Police said that I'm doing 87mph when my speedo reads 90mph 

[Boditza]

from what ive seen the faster i go the bigger the missread. at 50km\h gps says i have 49 but at 130km/h it reads 113km\h

[Andy B]

from what ive seen the faster i go the bigger the missread.  ....

it will do, it's how percentages work. 

[Lampynoiseboy]

Humberside Police said that I'm doing 87mph when my speedo reads 90mph 

Nice of them to pull you over to tell you 

[Andy B]

Humberside Police said that I'm doing 87mph when my speedo reads 90mph 

Nice of them to pull you over to tell you 

Too busy to pull me over, but they wrote a nice letter to tell me! 

[Lampynoiseboy]

Probably closing 4-lane motorways because someone lost an ariel.

Anyway, does this mean that those with XP migs, c/w calibrated speedos, don't have this issue? I assume they are calibrated for catching speeding motorists?

[Kevin Wood]

Probably closing 4-lane motorways because someone lost an ariel.

Anyway, does this mean that those with XP migs, c/w calibrated speedos, don't have this issue? I assume they are calibrated for catching speeding motorists?

Not necessarily. A calibrated speedo means nothing more than it has been certified when new to meet a certain spec. That spec might have been +/- 3 MPH, for example, meaning it doesn't read any better than a standard speedo. Tyre condition and ageing of the components concerned also comes into it of course.

GPS speed, on the other hand, if what's reported comes from the receiver chipset itself and is not calculated by interpolation by the sat nav, is probably accurate to 0.1 MPH in good signal conditions (4 satellites plus with good signal strength and a clear view of the sky).

Kevin

[Lampynoiseboy]

Probably closing 4-lane motorways because someone lost an ariel.

Anyway, does this mean that those with XP migs, c/w calibrated speedos, don't have this issue? I assume they are calibrated for catching speeding motorists?

Not necessarily. A calibrated speedo means nothing more than it has been certified when new to meet a certain spec. That spec might have been +/- 3 MPH, for example, meaning it doesn't read any better than a standard speedo. Tyre condition and ageing of the components concerned also comes into it of course.

GPS speed, on the other hand, if what's reported comes from the receiver chipset itself and is not calculated by interpolation by the sat nav, is probably accurate to 0.1 MPH in good signal conditions (4 satellites plus with good signal strength and a clear view of the sky).

Kevin

Would be nice to put one up against a proper speed camera to find out

[megaomega123]

I always work to the 10% +2 rule. However when my abs ecu was out I had to rely on satnav speed so the rule changed to just +2. Found the satnav speed to be mostly accurate but sometimes delayed.

[Dave DND]

Also depends on whether we are at war or not.

For those of you who remember the 1st Gulf war conflict, you may have encountered some very strange sat nav issues as the "spec" of the military satellites were altered to give some strange readings - the reason we were given by the manufacturers at the time was it was to prevent any GPS targeting being used against the US.

So bearing in mind that its also a US system, I think you would be very foolish to trust it, and would recommend using it only as a guideline of speed

[2woody]

it's actualy not legal to sell a car with an accurate speedo.

there's a formula, but basically expect your car speedo to over-read by up to 6mph at70