On driving vehicles with automatic transmissions in the advanced vehicle training course I found that all vehicles with automatic transmissions slow themselves down on hills so that they don't roll faster than a person walking.
However leaving the vehicles in neutral gear allows them to coast normally.
Attempted this bug with both the HMMWV, and 2-1/2Ton trucks.
Log into Arma, and turn off the vehicle helper.
Place an infantryman, and a vehicle with an automatic transmission at the top of a hill.
Hop in, turn it on, put it in drive, and slowly start heading down hill without touching any controls after your initial push.
Drive back up hill, and do the same in neutral.
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Hello UrstMcRedHead.
This is not a bug but a feature - https://en.wikipedia.org/wiki/Engine_braking
If in Neutral - the wheels have no resistance from the engine/transmission = car rolls down hill super ez
only resistance that occurs is aerodynamics + tire resistance to ground.
If in Gear (applies to manuals & automatics) - transmission & engine are engaged to the wheels = mechanical resistance = vehicle slows down/lower speed while coasting downhill
the slow down rate/max speed while rolling downhill depends on engine volume + vehicle weight + the gearbox ratio, those differ between various cars.