My Scion xB does the same thing until I stop for about 30 seconds to get the idle to really drop down to 650-640rpm then it will idle low while coasting. It maybe the cat in my case not quite getting up to temp but until I stop for a several seconds the idle is in the 800-900rpm range and burns more gas. Even when it finally does settle down it doen't get all the way down to the lowest burn rate/idle while moving and any power steering use raises the idle a little. It does seem to idle lower on hot days better than on cold days. Forget about nights because the lights increase fuel use. Come to think of it . . . it could also be related to battery charge too.