hspder

There are many theories out there... Unless there's somebody here that is "in-the-know" from inside one of the manufacturers, we'll never be sure.

There's just a couple of things we do know:

1. It's not a fixed amount; seems to be around 5-6% in most cases, with some corner cases on the 7-10% range. Which makes you wonder how much of it is on purpose and how much of it is poor QA. For the reasons I'm going to state below, my THEORY is that 5% is on purpose and the rest is indeed bad QA.

2. It's not for ALL manufacturers: both my Mercs are 100% accurate. So are GMs, Nissans/Infinitis and Volvos, according to magazine tests. BMWs and Porsches seem to consistently be 5-10% off too though, and specifically in the case of BMWs the 5% is indeed proven to be on purpose (because, with the right equipment, you can see that it's a setting in their onboard electronics -- in fact, you can see your 100% accurate speed in a BMW with a Scangauge connected to the ODB port!).

3. In many cases it can be adjusted with the right equipment too.

4. It is ILLEGAL in the US (Federal Law, DOT) for the error to be over 5%. If your error is above 5%, and the dealer can't fix it, you can even lemon-ize your car. Most people won't bother to do it, but this is a FACT. Problem is both BMW and Audi will refuse to fix it if it's below 10%, which mean getting a lawyer involved is your only option if you're in the 5%-10% range. OR do what I'm going to do which is to refuse delivery if it's in that range of error. :-)

Speedometer Accuracy Laws in Europe are FAR more lenient, to the point of being comical -- in the UK, for example, if you're driving 100kph and the speedometer is showing 116.5 kph that's perfectly fine... Ridiculous. But I think that may be why a MINI (UK-built) is far more likely to show a high speedometer error than an X5 (US-built). At least that's my theory, and I'm actually going to test that out soon. :-)

ltooz_a6_a8_q7

My A8 & A6 are right on the dot... Only the Q7 has the error.

24wheeler

I don't know where you get your information, but it is wrong.
British construction and use regulations stipulate a legal error margin of -0% and +10%. In practice most cars are intentionally built with a bit of positive error and most are within +2 to +5%.

hspder

From Wikipedia's article on speedometer; if it is wrong I suggest you edit it... It is mentioned in several places so make sure you edit it in all places and provide references.

Here are the relevant aspects of the article, i.e. the parts that you need to edit:

"
European Union member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers [4] [5][6] are similar to the UNECE regulation in that they specify that:

The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.

The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

[...]

United Kingdom

The Motor Vehicles (Approval) Regulations 2001[12] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However [...] the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
"

Jusforfun

Needs more tread depth or air pressure or both.
Does the error decrease when you travel West? JK

hspder

That's an urban legend -- changes in tread depth and/or air pressure have a 1% impact, nothing more than that. Just run the numbers... It wouldn't compensate for a 5% much less a 10% error.