I discovered a new behavior of the vehicle virtual engine simulation. The phyX update made it so that the engine is not decouple when not giving throttle. For tanks this is inconsistent however.
At fault is the current tank control scheme that cuts of engine power when turning. I’d like to remind you of this ticket, which is because of the same thing, but much more servere affect

http://feedback.arma3.com/view.php?id=20078

If you start driving and stop pressing W button the engine stays couple with the wheels and gearbox (essentially, the clutch is still “closed”). This can be easily noticed, because you still hear the engine sound even after letting go of throttle.
As the vehicle rolls and slows down the autogearbox reduces gears according to optimal rpm setting the same way as if there was throttle. You will hear how the engine is forced up in high rpm when a gear is changed down. For automatic gearboxed vehicles this is acceptable.
However, if you start turning (in a tank) the engine will be decoupled , which makes no sense at all. Either have it always decoupled without giving throttle, or always coupled.