Before 0.15 most of the vehicle can drive through mud area with no major problem. However since 0.15 mud has gone much deeper, and most map containing mud such as Endurodrome cannot be played(FailRace's Driven till Distruction results cannot be replicated), nearly all vehicles get stuck in mud. Mud pit demolition derby nearly all cars stuck in mud. Only serious off-road vehicle can drive through mud now. Pictures below show vehicles that can drive through mud in 0.14, Stuck in mud in 0.15.