Opening your exhaust leads to opening your wallet more often

 

you can have exhaust back pressure tested by a mechanic pretty easily but larger exhaust tends to increase the back pressure at lower revs because of loss of momentum of the exhaust gasses.
So please, before you spend any money make sure that you understand what you are doing, pretty much any book on exhaust design will give you the formula to size your exhaust for the engine speed that you want to run at and you just might find that your current exhaust piping is even larger then you really need if gas mileage really is your goal.

 

Easiest miles per gallon increases come form keeping the 4 corners aired up. Look to your load rating PSI and air up to you drive comfortably, not exceeding your max PSI rating.