Speedo accuracy

[njc021073]

I have my car in for a service today (04 2.0 TDi SE)and have a brand new S-line DSG 2.0 TDi Sportsback as a loan car.

One thing that has always annoyed me about my A3 is the innacuracy of the Speedo. At 80 mph it is indicating 86 on the speedo.

The Sportsback indicates 81 at a true 80 mph (much the same as my Passat).

How can it be that the speedo on my A3 is so innacurate compared to other VAG cars I drive.

Is there anyway of adjusting this?

[peterb]

All speedos will over read by anything up to 10%. I'm surprised that the loaner and your Passat are so accurate.

[PetrolDave]

To meet the Construction & Use Regulations the speedo must not under-indicate and is allowed to over-indicate by up to 10%.

Typically mechanical speedos are set at manufacture to over-read by 7%, since they tend to read lower with age due to wear and tear on the springs within the mechanical speedo.

Typically electronic speedos are set at manufacture to over-read by 3%.

Comparing the GPS speed from my Cyclops with the speedo on my 2004 S4, my speedo over-reads by less than that (31@30, 51@50, 71@70, 82@80, and that's all I'll admit to!!!).

[njc021073]

What confuses me is that my Audi over-reads a lot more (54@50 76@70 87@80) than my Passat (51@50 72@70 82@80) and the Audi loan car I had the other day - same model and age as my Audi which gave similar figures to my Passat.

All speeds were checked using my GPS and timing against KM posts using my Tachymeter watch.

Why would my Audi using the same technology be so different?

[Vic]

Neil

Try increasing your front Tyre pressures by say a safe 10%. Then check your speeds and report back. I wonder if tyre deformation is playing a part here? Less on low profiles, more on 'normal' tyres which would increase the rotational speed and hence the indication.

What make of tyres are on your SE?

Vic

[njc021073]

I thought about that too, as the loan audi had 17 inch wheels as opposed to my 16 inch, but I believed the 17 inch alloys come with lower profile tyres because of this, to give the same total circumfrence. Also the Passat has same wheel/tyre combination as my Audi.

As for over inflating, I already run them quite high to save on fuel (being a tight git)