Im running 235/70R15 Colway AT's on mine (got a 3 inck lift on it to make em fit) i get about 28 to the gallon and tend to hold steady at 60-65 on the motorway. the 205's you mention should be fine. its a 3% increase in diameter, so expect a loss in acceleration and fuel econony.
I dont know the ST tread patern, so cant commont on what that may do.
Matt