What b-man said. But, how are you with the current performance? I was getting 15-16 mpg at 65 mph in my '67 GTO with a TH400 and a 3.36 rear gear. When I changed to a 2.56 posi 12 or so years ago, the numbers went to 21 mpg at 75-80 mph. The car now sips gas, keeps up with the Hondas, and runs cool. Since it's a road-tripper and cruiser, I would NEVER think of installing a 3.36-3.55 gear in it again. Just saying....with regular gas at $5.49 a gallon where I am, I'll take the extra 6 or 7 mpg's and 15 mph all day long.
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Jeff 
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