Hi, I found that over speeds of 60mph the Speedo does not agree with my TomTom speed. Is as much as 10% lower than what the TomTom reads.

Mine reads approximately 5% high when compared to my Navman satnav and fixed road speed displays, something I have attributed to worn tyres.

New rubber should show your actual speed increase slightly to better match what you describe.

Correction, your tyres will have good tread or be wrong aspect fitting compared to original spec.

I got it the wrong way round there!

Nothing mass produced is 100% accurate which is why police vehicles use independent and routinely calibrated speedometers.

So can I trust my TomTom? I run winter and summer tyres, both are new, so I accept there will be different readings on the car Speedo.

I would generally say you can trust the speed on a sat nav, but I am surprised your speedo reads up to 10% slower. Generally speedos read fast, as they are allowed to read up to 10% fast, but should not understate your speed.

 

So for example I would expect your speedo to say 77 (maybe) when your satnav says 70 not the other way round.

If your speedo is indicating a speed slower than what you are actually travelling then you should get that looked at. As said above, a speedo can be as much as 10% out - so displays 80mph when you've actually going 73mph for example.

However if you're travelling along at 80mph and the speedo is only showing 70mph then that needs sorting.

I've used the hidden digital speedometer  (I forget the buttons you need to press on the climate control)  and  it tallies with my GPS.  By comparison my main speedo overreads by about 3mph at 70mph.

I have a 2012 VRS. What hidden digital speedometer? Never heard about this before, how do I find it?

Mike, This slight alteration in VCDS, is it a generic change that will suit all or possibly just specific to a car that thinks isn't quite right & how would you know?

 

If it's a case that it will make mine that little bit more accurate then it can't be bad. Could you please let me know what I need to change on controller 17?  

 

As a side note do you know if the lane change blinks can be increased from 3 to 5 in VCDS? This is another tweak i'd like to do if it's possible.

 

Warm regards

 

Mark

I use sat nav in a car fitted with the PUMA calibrated speedometer. Sat nav is not as accurate as it is credited. 2-3 mph out is not unusual. The unit is a Garmin 3597 BTW.

Oops, sorry my car Speedo reads 10% faster than the Tomtom.

My speedo consistently reads 10% higher than my Mio SatNav, running IGO8 software,  which, incidentally, agrees with the "Your Speed" signs which you sometimes see at the side of the road.  I, therefore, trust the SatNav's accuracy and have set it up to act as a digital speedo displaying the speed in large numerals.

 

Dave

Mine reads around 5mph more than the reading on my Garmin sat nav. 

I use sat nav in a car fitted with the PUMA calibrated speedometer. Sat nav is not as accurate as it is credited. 2-3 mph out is not unusual. The unit is a Garmin 3597 BTW.

Consumer GPS can be as much as 5% out.

I certainly wouldn't put my licence in the hands of a tomtom or garmin.

Oops, sorry my car Speedo reads 10% faster than the Tomtom.

 

Nothing to worry about then, that's perfectly normal. As above, the Tom Tom will be the more accurate.

I use sat nav in a car fitted with the PUMA calibrated speedometer. Sat nav is not as accurate as it is credited. 2-3 mph out is not unusual. The unit is a Garmin 3597 BTW.

Interesting. Sat nav should be accurate if driving at a constant speed on a road which is straight and level (ie no slope). Could you please check and see what the error if any is next time you are on such a road?