For a trip in Northern Spain this year I used wickiloc
. It allows you to access gps routes that users have posted. Zoom the map to the area you are interested in and any routes in the area will be displayed. They are categorised by method of travel, including 4x4 tracks, quad bike tracks and dirt bike tracks.
We managed days of over 100 miles almost completely off-road. 2 things to be aware of though:
just because someone has been there and posted the route does not mean it is legal.
Beware of the unexpected - some of the 4x4 trails were 50mph gravel tracks, some were VERY steep and difficult, especially with loaded bikes. Take a map, be prepared to follow your nose to get round something that is harder than expected and have a contingency plan.
Interestingly, the biggest navigation problem we had on the trip was when we decided to reduce the off road towards the end of one afternoon and follow a road marked on the 1:250,000 Michelin map. It ended up as a dirt track then completely disappeared. By the time we spent ages searching for the correct route, then had to take a long detour to get round on tarmac roads, it would have been quicker to continue on the planned off-road route.