But surely in gear with a closed throttle causes engine braking due to the pistons pulling a vacuum.
Question is, is the cost of the drag caused by engine braking and no fuel slowing the car down greater than the fuel used by an idling engine and the car freewheeling?
In other words a car in gear wit the the throttle shut will decelerate faster than a car coasting at idle and that lost energy has to be put back.