In many states, coasting is illegal, though it’s even more difficult to enforce than the seat belt laws.
The minimum brake specific fuel consumption is typically at about 90% throttle and at an engine speed corresponding to maximum torque. In other words, the best thing you can do is get into 5th as quickly as possible.
(Some trivia: During the Mobilgas Economy Runs, it was common to see many of the cars using their windshield wipers when it wasn’t raining. This was because the wipers were vacuum driven and the F/A ratio was leaned while they were in operation.)