Garmin Oregon 550 city navigator won't navigate correctly

[shirl]

Hi,
I am new to the GPS and have Garmin City Navigator on a micro SD card. No trouble accessing or using the maps. The trouble starts when I ask it to actually navigate somewhere. I began naturally enough by putting in a route I know well.
After planning my start and finish points, I then choose "automobile navigation", I am given the choice of minimise time or minimise distance. No matter which option I choose, it sends me the long way (sometimes with loops!) off main highways and expressways, along dirt roads and the direction I would choose if going the scenic way.   It reaches the destination eventually.
Am I doing something wrong? I have experimented now with a number of destinations and even put in points along the way on the highway to keep it on track but it still finds its way the wrong way! I really don't trust it at all now to show me the way in unfamiliar territory. Any solutions gratefully accepted. I have tried disabling the pre installed worldwide basemap and it makes no difference.   

[cembarnard]

If you are running software version 5.8+ then it is worth checking the settings in Setup > Routing... 
When a new Activity is selected this changes the list of entries in the "Avoidance Setup".
When Activity is set to "Automobile Driving" one of the entries you can disable is "Unpaved Lanes".