Garmin GPS

Can any one help? I have three Garmin gps units which appears to no longer have any preference for major highways. Since updating the maps about a year ago, they are inclined to direct me down narrow, winding, minor, single lane roads where you are lucky to be able to do 80kph over major highways.

One unit which I had not updated the maps did not do this but I finally got round to updating the maps at Easter and, you guessed it, it now thinks all country roads are capable of treavelling at 100kph.

For example, when recently travelling from Millicent to Mount Gambier - which are connected by the Princess Highway - the Garmin took me off the Princess Highway to Glencoe SA and then back onto the Princess Highway adding considerable time travelling on very minor roads.

Garmin Australia has been no help at all. Have any travellers overcome this fault as I can not rely on the Garmin when going into a new area?