Here are the EU regulations on speedometer accuracy as a PDF. In the US as of 2003 the maximum error is +5% for cars, but as with the UN regs, adopted by the EU, for bikes it may be more. These are federal guidelines, not state or provincial.
A nutshell:
European Union member states must grant type approval to vehicles. The ones covering speedometers are similar to the UNECE regulation in that they specify that:
* The indicated speed must never be less than the actual speed.
* 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.
The standards specify both the limits on accuracy and many of the details of how it should be measured during the approvals process, for example that the test measurements should be made (for most vehicles) at 40, 80 and 120 km/h, and at a particular ambient temperature. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle.
The UNECE regulation relaxes the requirements for vehicles mass produced following type approval. The upper limit on indicated speed is increased to 110 percent plus 6 km/h for cars, buses, trucks and similar vehicles, and 110 percent plus 8 km/h for two or three wheeled vehicles which have a maximum speed above 50 km/h (or a cylinder capacity, if powered by a heat engine, of more than 50 cc). European Union Directive 2000/7/EC, which relates to two and three wheeled vehicles, provides similar slightly relaxed limits in production.
Hope this helps.
BTW, I doubt 170 on an 848. Give me the GPS logs and I'll buy it.