"....You would be introducing all sorts of issues if you transferred a gpx file from Tyre direct to the Zumo ....."
This comment puzzles me. I have created literally hundreds of routes using Tyre and more recently MyRout-App. I have simply transferred those .gpx routes to a Zumo (various models) and have experienced very little difficulty with the routing of the GPS units. Once in a while, the Garmin gps unit will try to do something that is not on the original route (take a bypass for instance) but overall, it is rare. I am careful, however, to try and put 'waypoints' in the route that literally force the routing to use a specific road. ie. usually put a point at the beginning and the end of each road segment. Maybe that is the key but to be honest, I can create a route that has only 20-25 'waypoints' on MyRoute-App, transfer it to a Zumo and it will follow the routing, for all intents and purposes, 100%. I have attached a couple of examples. I created these routes in MyRoute-App and loaded directly into a Zumo and it followed them almost 100%.
I have been using Tyre since it first was introduced and I was a beta tester for MyRoute-App and can honestly say, I have experienced little issue with transferring .gpx files directly to Zumos.
I will be the first to admit that I don't begin to understand the machinations that go on inside a GPS but I am pretty comfortable creating routes, transferring, and then using them.
BTW- my comments apply equally to a TomTom unit that I still use frequently.