if i need engine braking, of course, coasting in gear is the best, with or without injector cutoff as needed.
if i don't need engine braking, of course, coasting in neutral is the best, with engine off or idling as needed.
any vehicle getting better mileage coasting in gear, would have to have some kind of over-running clutch system, and a very smart ECU, that idled the engine at the same time.
the only one i have driven that did this was an Audi turbo-diesel with the transmission in E.
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