Why is my speedo out ?

[jamiep91]

Why wouldn't my speedo be right with the standard gti wheels on ? Travelling down the motorway last night it was at least 5mph out on the satnav

[Top Cat]

Erm! every car in the land reads out, compared to a sat-nav all manafactures drop it a couple of percentage points. The faster you go the more it is out. 

Is this your first ever sat-nav. 

[stealthwolf]

You do realise about speedos and errors of margin?

Eg a speedo might have a +/-10% margin of error. That would mean when it reads 30mph, you could going as slow as 27mph and as fast as 33mph. Speedo has to be adjusted so that when the needle reads 30mph, you can never be going above 30mph (even if it means you have to go slower). Wheels and tyre sizes will affect this.

[rich83]

Erm! every car in the land reads out, compared to a sat-nav all manafactures drop it a couple of percentage points. The faster you go the more it is out.  

Is this your first ever sat-nav.  

So when i was doing 180 the other day.... i wasnt doing 180????? 

(N.B. this did not happen)

[swgti]

Hence why there's the "unwritten law" that you dont get a speeding ticket if within 10% of the speed limit, certian police cars have specially calibrated speedos as the stock item from the manufacturer isnt accurate enough in any way to force a convition in court.

True story, my mate (yeas really my mate not me) got off scott free with a speeding ticket after questioning if the car that stopped him was calibrated, it wasnt, it was actually a copper that told him to challenge it and it worked.

[E30Dom]

Check it against the digital speedo ;)

[GrayMK5GTI]

Also worth noting that a sat nav is ONLY ACCURATE if driving over a level road (ie not uphill or downhill) as it cant adjust for driving up hill (you may be doing 70mph but not covering the geographical distance the sat nav thinks you are)

Graeme

[chrisr763]

Also worth noting that a sat nav is ONLY ACCURATE if driving over a level road (ie not uphill or downhill) as it cant adjust for driving up hill (you may be doing 70mph but not covering the geographical distance the sat nav thinks you are)

Graeme

I fear im being stupid. But surely its got to be a very steep long hill to affect the geometry on the sat nav? 

[Hedge]

I think if you drive off Beachy Head then the last thing on your mind would be your GPS speed. 

[Hurdy]

Also worth noting that a sat nav is ONLY ACCURATE if driving over a level road (ie not uphill or downhill) as it cant adjust for driving up hill (you may be doing 70mph but not covering the geographical distance the sat nav thinks you are)

Graeme

I fear im being stupid. But surely its got to be a very steep long hill to affect the geometry on the sat nav? 

Depends on the nav unit. Some GPS units can take into account altitude in calculations too.

Erm! every car in the land reads out, compared to a sat-nav all manafactures drop it a couple of percentage points. The faster you go the more it is out.  

Is this your first ever sat-nav.  

Not entirely right. My missis had a Honda Jazz and at 90mph on the speedo it was doing exactly 90mph on the portable nav unit.  

However my R is typical VAG and out by 4mph at 60mph on the GPS and a full 10mph at 120mph on the GPS (track).

[jamiep91]

Erm! every car in the land reads out, compared to a sat-nav all manafactures drop it a couple of percentage points. The faster you go the more it is out. 

Is this your first ever sat-nav. 

I know every car is out and it's not going to be 100% and its not my first day nab funnily enough but my other cars have never been this far out think it was 80 satnav ,88 speedo

[GrayMK5GTI]

Also worth noting that a sat nav is ONLY ACCURATE if driving over a level road (ie not uphill or downhill) as it cant adjust for driving up hill (you may be doing 70mph but not covering the geographical distance the sat nav thinks you are)

Graeme

I fear im being stupid. But surely its got to be a very steep long hill to affect the geometry on the sat nav?  

A fairly steep hill to make a 1-2mph difference. Some of the hills on the M6 show the sat nav lose 1 or 2mph.

[MC71]

Erm! every car in the land reads out, compared to a sat-nav all manafactures drop it a couple of percentage points. The faster you go the more it is out. 

Is this your first ever sat-nav. 

I know every car is out and it's not going to be 100% and its not my first day nab funnily enough but my other cars have never been this far out think it was 80 satnav ,88 speedo

10% sounds right as far as a VAG discrepancy goes! That's what mine usually says anyway. 

[homesqueeze]

The fastest I've taken mine is 160mph on the speedo. On my satnav, the speed actually shows 150mph. Quite a big difference at these speeds!

[james]

If u got a polour thing, it tells u the 100% correct speed.