When driving in a vehicle in ARMA3, if you attempt to drive uphill at all, your vehicle nearly grinds to a halt. Often driving no more than 15-20km/h. This happens no matter how fast you're driving when you actually hit the slope itself.
This same issue was in ARMA2 and it proved to be quite an issue in the long run. Operations and missions were slowed down a lot, or our creativity was stumped because we had to choice to take roads, or go INCREDIBLY slow. Almost to the point of a snail's pace.
In reality, these vehicles would move far faster up these hills than shown. Especially tracked APC's and Main Battle Tanks.