Factory manufacturers have spent millions of dollars trying to get any advantage when it comes to mpg. They spend millions of $ and tons of time in wind tunnels making bodies more aerodynamic so you get better mpg. If vehicle weight made a huge difference, they would spend more time on a scale and less time in the tunnel....its all about aerodynamics, much more than weight. The weight factor comes in when you are talking about puny engine like aprius that simply cant push more than 2500#s. In a V8 with lots of torque, its the profile of the body lines that matter most.
