Now I like to drive legally but also like to travel as fast as I can between locations . To this end satnavs - my latest is a Garmin - are an invaluable aid to give you a better ( I am not saying perfect ) idea of your true speed .

My 2008 Scout Tdi revealed that its speedometer was about 5 % over optimistic when measured against the satnav which was certainly better than my mark 1 Octavia which was nearly 10% adrift .

Having done nearly 9K in the Scout I noticed today that the two readings are now identical !

The Skoda , being a Skoda , has not needed any attention from a garage , apart from having a tow bar fitted whereas the Garmin has had 2 firmware revisions or updates when connected to the internet .

I can therefore only conclude that the Garmin has been " altered " - in the interests of ? economy or ? those who feel that we still drive too fast even when we obey the speed limits . Have you noticed those illuminated signs that say slow down when you are doing 27 mph in a 30 mph speed limit zone ?

Has anyone else observed this " phenomenon " ?

can't imagine the sat nav to be wrong but I suppose its possible, if you can try another sat nav in your car at the same time see if they match.

The trouble with those road signs is that they're never calibrated properly.

My satnav always shows a difference between observed and 'true' speed, my fabia and golf were about 4-5 mph out at 70mph, whereas my GPS is 7mph out at 70mph, very over-optimistic. But then 10% is legally allowed and i suppose it could save points on your licence.

You don't suppose someone has sued the Sat Nav makers after getting nicked speeding when relying on the speed output of the Sat Nav?

As these things are sold in the "land of the sue" - USA.

I'd expect that there are plenty of legal disclaimers within the packaging and software license (that you get with your Sat Nav) saying that speed is for "representation" and may not be actual speed.

SatNav is correct, Speedometer is wrong...

You can see your true speed using climatronic (if you have it):

http://briskoda.net/octavia-ii/new-octavia-tips-tricks/55750/#post637217

See Main channel: 19 - Data Motor.

Surely as the tyres wear down the wheels must rotate that bit faster. As a result the car will display a speed that has changed from when the tyres were new. The only way to refer this changed speed to real life is to compare it with a known true reference - the GPS.

Surely as the tyres wear down the wheels must rotate that bit faster. As a result the car will display a speed that has changed from when the tyres were new. The only way to refer this changed speed to real life is to compare it with a known true reference - the GPS.

Speedo accuracy does change as the tyres wear but it is the other way round to the reported effect - as the tyres wear down you go less distance per wheel revolution so the speedo will read even more optimistic.

jawsnpaws - have you put new tyres on recently?

You may find the speedo of your car more accurate than you thought, and the satnav updates were to fix errors in its calculations of speed. The only way to tell is to use another vendors satnav side by side and compare them.

I have a garmin sat nav and love it.

The firmware updates are just fixes to improve the usage.

At speedo 70 Sat nav reports 65

At speedo 80 Sat nav reports 76.

I also have an older GPS which I use for hiking and sailing. It reports exactly the same speed as the satnav.

Sat navs sold commercially have an error of +-5%. Military had +-0.001% margins - but then they cost a lot more than £100!

I really wouldnt rely on it *that* closely if you're using it to get as close to the acpo penality speed guidelines without being illegal.

There are many documented news stories of people who've regreted it.

Current GPS receivers are very accurate!

Typical speed error is less than 0.5 Mph in most cases (with good reception), because of integration and other tricks.

gpsspeed

http://nujournal.net/HighAccuracySpeed.pdf

when i first got my octy it was 5% out like all my other cars, once i had the maxidot fitted the speedo is only 1 or 2% out and you only notice its slightly out when you get up to 70+

Thanks for all the interesting replies .

For clarification , it was a new car in May that has now covered 9K .

My real point was not so much the accuracy but the fact that something has definitely suddenly changed by a factor of 5% , and I can only see that the GPS must have been altered .

have you got another device say a talex lying around that you could plonk on the dashboard and go for a quick drive with?

I have a garmin sat nav and love it.

The firmware updates are just fixes to improve the usage.

At speedo 70 Sat nav reports 65

At speedo 80 Sat nav reports 76.

I also have an older GPS which I use for hiking and sailing. It reports exactly the same speed as the satnav.

These figures match those shown by my Garmin IQUE 3600.

When I was driving in Germany I took my car up to 120 on the speedo. Sat nav reported this as 108. A very neat 10% error.

Thanks for all the interesting replies .

For clarification , it was a new car in May that has now covered 9K .

My real point was not so much the accuracy but the fact that something has definitely suddenly changed by a factor of 5% , and I can only see that the GPS must have been altered .

You could use a measured mile to see what the accuracy of your speedo/tacho is?

There is a very simple way to get your true speed, without using a GPS, or playing with the climatronic.

Just switch on the cruise control, and clear the average speed on the MFD.

Then you'll get on the MFD the exact speed you are driving.

There is a very simple way to get your true speed, without using a GPS, or playing with the climatronic.

Just switch on the cruise control, and clear the average speed on the MFD.

Then you'll get on the MFD the exact speed you are driving.

Which isnt calibrated, so will still carry an error margin.

Everything in the trip computer is very well calibrated (if using standard tires ofcourse).

Only the arrow of speedometer is altered.

You can see that mileage counters are absolutely correct.

Try what I'm saying and you'll see the result .