Having been involved in the vehicle navigation industry since 2002, the difference between the speedo reading and the GPS speed is one of the most common questions I get asked. The GPS speed is accurate to about 0.2 kph when measured at a steady state so it's pretty accurate.
I've just done an 1800km trip in my RCZ and had the GPS running for most of the trip. One of the first things I did was to calibrate the speedo reading against the GPS. As others have said, the in-car speedo always reads high so you should never get a speeding ticket if you drive to your speedo. My experience is that the analogue speedo generally indicates up to 5% faster than the actual speed; that is, at a real speed of 100kph the analogue speedo will read around 105kph. In other words, if you drive with your analogue speedo on 100kph your car is actually doing around 96kph. The difference between the two speeds will depend on your car and the tyres installed but generally the difference is around 4 - 5 %. As your tyres wear, the difference between the two speeds will change slightly as your tyres get smaller.
What surprised me with the RCZ is the difference between the built-in digital speed display and the analogue display. The digital display shows the usually expected value of 5% above the actual speed (i.e. 105kph when traveling at 100kph) but the analogue display shows higher still. If I drove at 100kph actual (GPS) speed, the digital display showed 105kph while the analogue display showed 108kph. My last two cars had both digital and analogue speed displays and both displayed the same value so I was surprised to see quite a difference between the RCZ's speed displays.
In Australia, the police measure the real speed so you won't get booked for traveling at the speed shown on the GPS. In NSW they allow up to 10% tolerance for error so even if you go a little above your GPS speed you shouldn't get booked.
