Here's why:
* Headers: Headers improve exhaust flow, which can slightly increase horsepower and torque. *If* this leads to more efficient engine operation at lower RPMs, you might see a minor improvement in fuel economy. However, the increased power often tempts drivers to accelerate more aggressively, negating any gains.
* Dual Exhaust: A dual exhaust system primarily improves exhaust flow as well. Like headers, the improvement is often small and doesn't directly translate to better fuel mileage. The main benefit is usually improved sound and potentially a slight reduction in back pressure. Again, any potential fuel economy improvement is likely to be negligible and easily offset by driver behavior.
In short: While theoretically possible under very specific circumstances, the impact of headers and dual exhaust on fuel economy is usually too small to be noticeable. The cost of these modifications usually far outweighs any potential fuel savings. If improved fuel economy is your goal, focus on other modifications or driving habits.
