Distances Vary Between Basecamp & GPS

Former Member
Former Member
I have created my first route in Basecamp which appears to be Ok and the distances is stated as 66.5 miles. I have successfully transferred this to my Montana but the distance it shows is 55.5 miles. I have deleted the GPS route twice and reloaded from Basecamp and the distance is always 55.5 miles.
I initially created two routes and ,I thought, successfully joined them. The route appear to be similar but it is quite difficult to check on the Montana. I am reasonable sure that the Basecamp map is showing the correct details.
Any help would be much appreciated>

I should have said that I have looked for threads with similar problem without success

Regards

David