How accurate

[mds141]

Just out of interest anyone experimented with speedo accuracy using a sat nav? When i brought the valver home from southampton noticed that the speedo and tomtom were in agreement never had this on a "modern" car my 407 is approx 5% out and the Honda Firestorm even more.

[Dollywobbler]

I've never had agreement.

At a speedo 70mph the 2CV's actually doing 63 and the BX about 65. Mind you, I think Sat Navs differ a bit too.

[Vanny]

Modern cars are normally under by 4mph at 70mph, its a legal thing i think, bloody annoying when your in a car with a calibrated speedo and you KNOW everyone is travelling below the limit.

[Wooscary]

Went past one of those "your speed is" signs the other day. Speedo displaying 5mph under what the sign quoted.

[Philip Chidlow]

My XM speedo reads 76 when I'm doing 70, and the BX is very similar.

[Defender110]

I thought sat navs calculated an average speed between set points not a continuous actual speed?

[Philip Chidlow]

If that was the case it wouldn't continuously update and react in real time as opposed to giving your speed retrospectively, surely?

[mat_fenwick]

Depends how close together those two points are!

It's interesting how some people assume a sat nav is always correct... a bit like the general assumption that a digital display is more accurate, especially if it has lots of decimal places.
Anyone got (or can lay hands on) more than one sat nav? You could drive along and get a passenger to compare the displayed speed.

[MULLEY]

Speedo reads approx 70, satnav says 63, & i thought that everyone else was driving too quickly before realising my slowness

As per what Phil say's, it seems to show your speed changes in milliseconds & shows no signs of a delay if its between points, so i'm not exactly sure how it actually works. It must be more accurate than a cheap french speedo head, there's waaaaaay to much plastic in there to be any good imho

I tend to just drive the car at 3000ish odd revs when crusing, get good economy & because the speedo is showing 70, i'm fooled into thinking that i'm not going that slow, so can stick at that speed without any probs, been upto Newcastle & back & got 58mpg sticking to those revs.

If i'm driving on any other roads with other speeds, i just adjust my speed to be more than what the limit is, satnav say's 50 when speedo is showing 55, so i just remember the various one's, so as not to be a mobile chicane & oldman duffer driver

[Defender110]

so as not to be a mobile chicane

''MOBILE chicane'' Well that would be an improvement on your entrance to the CCC Midland gathering

[Way2go]

Just out of interest anyone experimented with speedo accuracy using a sat nav? When i brought the valver home from southampton noticed that the speedo and tomtom were in agreement

My GTi is also spot on the same across the speed range compared to my Garmin.
I was surprised & pleased initially as I was expecting the speedo to read slightly fast but of course it does mean making doubly sure going past speed cameras as theres no "buffer zone"as a result.

[kiwi]

Just out of interest anyone experimented with speedo accuracy using a sat nav? When i brought the valver home from southampton noticed that the speedo and tomtom were in agreement never had this on a "modern" car my 407 is approx 5% out and the Honda Firestorm even more.

I started a similer thread to this before, it is somewhere in the archives.

Started this debate on a couple of Kiwi and Aussie forums...geeze the amount of finger pointing nanas around on the subject telling me I was wrong, I should not have a licence etc etc etc

Basically of our two BXs there is a differance of 10kph displayed on the speedo @100kph! Kind explains why the TZS gets there quicker, covers less Kms and has a heavier fuel consumption

Suggestions to replace the Dial, the cable have been in effective in resolving the issue in fact changing the dial made it worse! It seems the current Dial was calibrated but not perfect. The problem may lie in the head attached to the gearbox. I also noted a differance regarding temperature as to accuracy!

Still raises some strong debate though with the advent of widespread Sat Nav systems being stuck in cars.

Best one for me is the work van I discoverd it was reading 10kph high which is great now I know this as 110kph feels so naughty with the nanas who bang on about speeding of course the other side is how devilish it is to be reading 100kph and cruising at 106kph on the sat nav

[MULLEY]

so as not to be a mobile chicane

''MOBILE chicane'' Well that would be an improvement on your entrance to the CCC Midland gathering

Mind you, i thought all auto n/a's were mobile chicanes due to the maaaaaasive power available through the slushbox

[Defender110]

Touche

[Gibbo2286]

Speedometers and odometers will of course read differently simply by a variation of road wheel diameter, low profile tyres, worn tyres, odd makes, all can be different. Gibbo.