ttMaps is the site and it is free
The load the program and few files from the TomTom onto the SD card. When I need the offroad ability, I switch off the GPS, push the card into the slot and switch on. This will now give me offroad tracking. I downloaded the ecw maps for free and found the 250,000:1 is ok for touring but small areas require 25,000:1 which works wonders. You can switch between maps with a touch of the screen.
Look at this thread
tom tom 4wd maps