The use of in-car navigation systems has rapidly grown over the last years. For freight transport, the emphasis on fastest routes is not always in the interest of the truck driver and causes negative externalities when the routes run through built-up area. This paper reports the results of a field test that was conducted to test a new navigation system for trucks. A sample of 100 truck drivers participated in the experiment where they used the new system first in a tracking mode only and next in a full navigation mode for a period of two and a half months in total. During this period drivers kept a diary where they indicated the times and reasons why they deviated from routes suggested by the system. We analyze the diary data and GPS data logged by the system to investigate compliance with and influence of the new navigation system on route choice. The results suggest that the new navigation system for trucks has significant impacts on routes, the most important of which is a shift from use of smaller roads towards larger roads with beneficial consequences in terms of both efficiency and environment. Deviation from suggested routes occurs relatively frequently and for different reasons by drivers of heavy and lighter trucks. We conclude that development of dedicated navigation systems for trucks would serve the interest of the freight transport sector and quality of environment at the same time and more so when route advice is differentiated between heavy and lighter transport.