When I put the 2.56 rear in my '67 GTO ragtop (originally 3.36) it changed the car. The car has always been my 'cruiser' car (my '65 hardtop is my hotrod) and now that it cruises effortlessly at obscenely high speeds, it is more fun to drive than ever. Getting over 20mpg at 75-80 mph, I've taken to driving it on long road trips, sometimes 1000 miles in a long weekend out of town. Engine noise is way down, temperatures are way down, and fuel economy and road speed are way up. I can cruise in style and keep up with traffic now. Ideally, with my TH400/400 engine, a 2.93 would be optimum for quickness and top end, but the 2.56 posi was given to me, so that's what I'm running. I can't imagine putting the 3.36 back in, and going down to a 65mph cruise speed at 15mpg.
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Jeff 
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