weight is not resistance. a car with even a small ammount of force and huge weight will accelerate forever if there's no friction. it's basic physics. f=ma (net force = mass times X Acceleration) where the net force is the force applied minus the force of friction and any other forces against it. the reason that the 6000lbs of ectra weight make a car slower at high speeds is because it adds extra aerodynamic resistance. the extra weight is just the force of the wind pushing down on the car, it doesn't have to move 6000lbs.