Stand on the RAT or the Tank without falling off while a friend is driving at full speed. On the RAT, you can be in the gunner seat; just get out and walk the deck of the car without falling (at top speed). Or, take the Tank and if a friend (or enemy) is driving, you can get on the back of the Tank's bumper. Make your way around to the front or the tank by skimming the side of it.