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Anyone else notice a 5mph or so discrepancy between indicated road speed and a sat nav?

If I go 60 my sat nav is telling me 55 - I believe the sat nav.

Can this be changed using elmconfig?

Sent from my Nexus 4 using Ford OC mobile app

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Speedometers always over read, they have to by law.

They can be anything up to 10% out, and the accuracy varies depending on your tyre pressures and how much tread you have left.

They are set to over read so that even with the inaccuracy you are always under the limit.

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I have a scanguage in my focus for coolant water temp and that has MPH on it and it's roughly 10% faster than speedo and scanguage is bang on through engine ecu

Just gives you that little bit for speed cameras I look at it that way. Police cars on the other had get recalibrated after purchase so they are spot on for getting people's speed

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If you go into the test mode on the dash and scroll to the road speed, its more accurate than the speedo but i dont know if its 100% accurate as a GPS would be

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My old car was 3mph out at 70 but only 1 out at 30, it just seems a lot!

45 = 40 so I'm 12% out, which I thought was the legal upper limit for over reading.

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I think they have to be accurate within 10%, but can never under read (i.e. you are going faster than what it says).

They have to account for the fact that the sensors are not perfect, tyre circumference (affected by pressure/tread) and potentially parallax errors.

So the tolerance is always set to give you a faster read out than what you are actually doing.

Sat Navs are genuinely more accurate but are delayed in their reading as it has to calculate the distance over a longer period of time.

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I have a scanguage in my focus for coolant water temp and that has MPH on it and it's roughly 10% faster than speedo and scanguage is bang on through engine ecu

Just gives you that little bit for speed cameras I look at it that way. Police cars on the other had get recalibrated after purchase so they are spot on for getting people's speed

The police have other technology for capturing speed too, I believe.

But this would explain the 10% rule that you often hear about - You won't be stopped unless you're 10% over the speed limit. Which obviously must mean 10% over what your car is telling you what you are supposed to be doing.

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I think they have to be accurate within 10%, but can never under read (i.e. you are going faster than what it says).

They have to account for the fact that the sensors are not perfect, tyre circumference (affected by pressure/tread) and potentially parallax errors.

So the tolerance is always set to give you a faster read out than what you are actually doing.

Sat Navs are genuinely more accurate but are delayed in their reading as it has to calculate the distance over a longer period of time.

Not to mention that there are always the plonkers who immediately go out and stick a set of 19inch rims on and slam it on the deck. If it was super-accurate, it wouldn't read the speed right in that scenario, especially if your car started out with 14inch rims.

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Not to mention that there are always the plonkers who immediately go out and stick a set of 19inch rims on and slam it on the deck. If it was super-accurate, it wouldn't read the speed right in that scenario, especially if your car started out with 14inch rims.

:whistling:

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The police have other technology for capturing speed too, I believe.

But this would explain the 10% rule that you often hear about - You won't be stopped unless you're 10% over the speed limit. Which obviously must mean 10% over what your car is telling you what you are supposed to be doing.

I drive articulated/18 tonne lorries on the odd occasion. This is a common discussion.

Tacographs are calibrated periodically, every 2 years I think. They have much smaller margins of error in the Speedo. Coppers never bother you at 10% over the speed limit. That's why you see lorries on their limiters on dual carriageway with no problems. That's 10% over the actual limit. 55 in a 50.

Sent from my KFTT using Ford OC mobile app

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My speedo also indicates 75 when the satnav shows 70 so if I'm safe up to 77mph it would be showing 82.

I rarely go above [limit + 5mph indicated] though and then only to shorten overtaking time.

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My speedo also indicates 75 when the satnav shows 70 so if I'm safe up to 77mph it would be showing 82.

I rarely go above [limit + 5mph indicated] though and then only to shorten overtaking time.

What really annoys me are people who speed through built up areas. I don't care if somebody wants to do 95 on the motorway, but doing 45 in a 30, that's unforgivable, IMO. There's one simple argument for this - what if a child ran out in front of you? What would you do? Yet I always have people tailgating me, flashing their lights, overtaking in 30 zones, sometimes at school times. Madness! How do you get that message through to people? If you hit a child, you'll kill them. How would you live with yourself then?

Sorry for the rant, it's just something that really annoys me.

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On that topic what irritates me are people who dawdle along at 45 on a clear straight road then enter a 30 limit and continue at 45. I just have to wonder what's going on in their brains.

I thought the exact same thing after reading MiltyG565's post, lol.

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On that topic what irritates me are people who dawdle along at 45 on a clear straight road then enter a 30 limit and continue at 45. I just have to wonder what's going on in their brains.

Yes, well, I wasn't even going to start on those people! Plenty of the "Mr.45s" around here - mostly farmers, and generally older folks too. They seem to have very little regard for holding up traffic or breaking the speed limit.

It's odd how there are so many of them, though, and they all trundle along at 45. Is that part of their genetic code, or something? Is it luck of the draw?

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My speedo also indicates 75 when the satnav shows 70 so if I'm safe up to 77mph it would be showing 82.

I rarely go above [limit + 5mph indicated] though and then only to shorten overtaking time.

Exactly the point I was trying to make! (Assuming the 10% rule is correct)

I stick to 2-3mph above the limits. Doubt there would ever be an issue with the police or speed cameras.

Sent from my KFTT using Ford OC mobile app

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mostly farmers, and generally older folks too

Hey, watch it! I'm nearly at my threescore and ten :D

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On that topic what irritates me are people who dawdle along at 45 on a clear straight road then enter a 30 limit and continue at 45. I just have to wonder what's going on in their brains.

I was unfortunately following one of the 45mph club for ages just could not over take him. I did have a good laugh when he went through a speed camera

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