This is a classic question. In an AT it's Deceleration Fuel Cut-Off (DFCO) and high drag in gear versus idle gas consumption and low drag in neutral. Basically, with the usual significant difference between the drag in neutral and in gear, it's more efficient to be in neutral for deceleration. But, if you have excess speed such that you'd need to slow down faster than you would in neutral you keep it in gear for as much time as you need, trying to make sure it's in DFCO. Of course, the ideal is that you use Driving Without Brakes: get your foot off the gas and begin deceleration in neutral as early as possible. With perfect foresight you can take your foot off the gas much earlier than you can when coasting in gear. Once you've N-glided, gears feel so draggy.