@Ian-Syphas Hi Ian, of course I don't know the specific device you have, but I can give a general explanation that could explain this. on many satnavs calculations are still done on-board using locally available sat-nav data. Online services such as MRA Navigation, Waze, Google etc. all use live updated traffic data. The frequency of data updates plays a large role in the difference in travel time. For example, if the computer uses "guesswork" based on average delays etc. you could get a wholly different outcome than with a computer that uses live information. In general, the latest estimate from online services is the most accurate. As @Nick-Carthew pointed out, your satnav will update accordingly while en-route.