This is to do with how the miles are computed, i.e. road miles versus "bird miles".
The best solution for more accurate long-distance trips is to cut the trip into a series of short trips.
You could also try using Google Maps to calculate an accurate journey time.
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The Nuvi will recalculate the route using its own computations - it can vary if you have it set for shortest distance, fastest distance, avoid toll roads, etc.
Usually the GPS device will calculate the more accurate journey time, as it is designed to take into account things such as the time of day (rush hour), etc.
You could try tweaking some of the settings on it, although I suspect you should already have it set up pretty well for your journeys so don't change too many settings.
You should try calculating the journey time using Google Maps or other online journey calculators to see which result it is closer to.
Hi, just checking in on your question, do you need any further assistance?