Pipe Sizing Calculator
Use this for a VERY accurate calculation of your diameter size.
You don't need HP for fuel efficiency. You DO want a properly sized exhaust, with a SMOOTH flow in your exhaust.
Mandrel bends, straight flow mufflers, high-flow cats. THAT's what will give you the benefit.
For example, I had a turbocharged mustang that many people suggested I went with dual 3" exhaust. When I ran it through the calculator, I found that a SINGLE turbo with SINGLE 3" exhaust, would be enough to reach 600hp (remember, turbocharged changes the dynamics). I saw an increase in HP, AND I was getting 22 mpg if i could keep my foot out of the boost. All this and it was about 550hp.