Reproduction:
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A good way to see this problem is to take a tracked vehicle (Slammer/T100) and try to drive in a continuous circle by holding forward power and turning at the same time. The vehicle will keep slowing down until it is turning on it's own axis rather than maintaining a constant radius circle (as it does in Arma2).
You can also see the problem by simply driving around and watching the speed indicator/ behaviour on the vehicle.