John, about 3 years ago I changed out the 3.36 rear end in my '67 GTO convertible (TH400, 400 engine, 87cc iron heads (9.3:1) and 068 cam) and installed a freebie 2.56 posi that a friend had. I got the same results. I average 20.5 mpg at a steady 75-80 mph, and the car is smooth and quiet at those speeds. Passing power is excellent, and it'll hold 2nd gear until 105-107 mph at WOT. Off the line, not as responsive as the 3.36 gears, but still just fine. Well worth the trade off. I believe that the Quadrajet has much to do with it: it's a super efficient carb at light throttle cruise. I used to be 3,000 rpm at 70 mph....with the 2.56 rear, it's a tick over 90mph at 3,000. Car runs nice and cool, and gets the same MPG as my modern 4Runner, so I drive it a lot more on long trips now, too.
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Jeff 
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