Contact the makers of Tyre and / or try their forum:
http://www.tyretotravel.com
What I guess you are seeing is a mass of small 'breadcrumbs' or tiny individual dots that the software creates to fill in the gaps between shaping or waypoints. For users whom create their routes in Garmin BaseCamp or Mapsource software, the breadcrumbs are invisible. You have created your routes in Tyre, a non-Garmin piece of software, which you've then sent to your Garmin GPS device, where - for reasons unknown - the breadcrumbs maybe now appear.
Besides the chance of simple incompatibility creating the problem, there are a number of other possible causes. I'm not familiar with Tyre but here are some guesses or ideas for you to try:
1. Create a small, say 50 mile route in Tyre and send it to your Garmin device. Do the breadcrumbs appear? Yes? I suggest the simple 50 mile idea only to keep things simple whilst you muck about and try fixes
2. In what format is the route being exported from Tyre into the Garmin? It should, I think be .gpx
3. Can you alter the route's type in Tyre? For example, I read that you can create things called Tyre Tracks or something like that. If so, possibly a Track (or whatever Tyre calls it) carries the breadcrumbs with it? Have you tried mucking about in Tyre to see if you can alter the type of route to see if that makes a difference to the result?
4. Have you tried loading either BaseCamp or Mapsource onto your PC, then viewing the Tyre created route there? If not, try it. What do you see? Have you tried exporting the Tyre route from your PC to your GPS device via BaseCamp or Mapsource? What happens?
5. What happens if you display the Tyre route in BaseCamp or Mapsource on your PC, then hit recalculate and then export the route to your GPS device?
There really are so many variables that it's difficult for someone, not intimate with using Tyre and Garmin together, to help you. That being said, there are enough people (yourself included it seems) who do use Tyre and Garmin together without too many problems, to guess that the cure should be simple.
Report back with your news, please. Definitely let us know if you find a good / reliable answer to the cause of - and solution to - your problem. I'm sure it will help others.
Richard