Speed Limit: What Does It Really Mean?

Hi everyone...
It might be a silly question, but I'm not entirely clear on the definition of 'top speed.'

Is the top speed of a car (specifically, the top speed listed in the vehicle registration document) the speed that the car should be able to accelerate to on a straight road under ideal conditions, or is it the speed that the car can reach under absolutely optimal conditions (e.g., going downhill with a tailwind and someone pushing it from behind)?

Hi Tito,

The speed is measured in the horizontal plane under windless conditions. The temperature and air pressure should be the same as those used during the DIN performance testing.
At what vehicle load capacity... I have no idea whether it's just with the driver or half-loaded/fully loaded, but that probably won't be a decisive factor here anyway.

Best regards, Rainer.

I'm just wondering because last night I took my Golf4 TDI with 110 horsepower on a short highway test drive, using the air conditioning as a kind of 'diagnostic tool.'
According to the speedometer, I can reach approximately 210 km/h going downhill (and for a long distance).
My car's speedometer showed a speed of only 192-194 km/h at the same time.
According to the speedometer, I can reach approximately 200 km/h on a straight road, but with the air conditioning on, it's only just over 180 km/h.

Now I'm a bit unsure which one to trust (the air conditioning or the speedometer), or whether this is normal.

Hi, it would be best if you went to the ADAC (German automobile club). They can check the difference between your speedometer and your actual speed for a small fee.
To do this, the vehicle is accelerated on a dynamometer using its engine, and the actual surface speed of the rollers is determined at speedometer readings of 50, 100,... km/h.

Best regards, Rainer.

For comparison:

According to the speedometer, a Golf III Variant with 110 horsepower should be able to reach 205 km/h on a flat surface. That's already close to the officially registered top speed of 187 km/h.

Sometimes the climate control system doesn't display the correct temperature. I've heard that before.
My climate control display shows a reading that is 5% off, which is definitely correct.