Here is a hugely useful tool courtesy of Devon County Council.
It's an interactive map of all the public rights of way in Devon, including all legal byways.
Why should you care?
Because you can only drive your Land Rover off road on byways & unclassified roads. Footpaths & bridleways are no go areas for motorised vehicles.
So here's the link you want to bookmark: Devon Public Rights of Way Map.
You are looking for red byways marked on the map, cross reference them to your OS map or GPS, and away you go.
If you know of any other similar online maps for other counties, post them in the comments below.