YEARS ago, like over 40 years ago... I took my 63 GMC Sub with 305/SM420 (3.73 rear gear) to Midas for a dual exhaust setup. I was getting 11 mpg before with the single exhaust setup as they all came from the factory this way. The dual exhaust setup increased my gas mileage to over 13 mpg without doing anything else! Gears and transmission swaps are expensive- going to a dual exh setup gets you the most bang for your buck, in my opinion. NOTE: I had my pipes run between the frame rail and body,, and dump out at a 45 degree angle before the rear wheels rather than up over the rear axle and out the back, so I am sure this helped also with having less pipe. The mufflers I had Midas install were just a simple "turbo" style muffler (non-restrictive in nature) and sounded good without being too loud. Pipe size was a bit bigger than original stock size at 2.25 inch, but nothing crazy.
For what it's worth............
