[geeteeohguy]

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.

[north]

Good to hear I'm not alone, never thought I would add a bigger engine with an extra 100horses and a much bigger carb and end up with 25% better mpg!

I'm just starting to drive my sprint firebird with a Qjett ohc, wonder how it will compare? 150 less cubes but 3.55 gears.

There was a car life comparison test back in 69 that compared a 69 lemans, Buick special with a small v8 2bbl against a 4bbl chevelle, the chevelle was much faster and got significantly better mileage to boot. I have it somewhere.