About 10 years ago, I blew up the original 3.55 posi in my 66 GTO. Needing to get it back on the road, I put in the only axle I had available...a 2.73 out of a 71 Skylark. I HATED it with the 4 speed. The car didn't feel like a muscle car anymore. Sure, it would cruise down the freeway at low rpms, but it felt so sluggish off the line, it just wasn't fun to drive anymore. Fortunately, a couple of months later, I scored a 3.23 open rear from a 66 Cutlass (for free) and that is what is in the car now. Someday I will rebuild the original housing and have a 3.55 posi back in it....
My 67 full size wagon came from the factory with a 2.56 rear. It would rip down the highway. I put a 2.73 out of a GP parts car in it for a while (cause it was free, and gave a little better acceleration than the 2.56), and then I put a 3.23 in it when I converted it to a 4 speed. I wouldn't want to drive one of these cars with a 4 speed and anything numerically lower than a 3.23. I don't have a problem running 3500 rpm down the highway. I don't do a lot of long-distance highway driving, but when I moved to Utah, I drove my GTO 75 mph all the way.
For me, gas mileage is not my biggest concern. These cars were intended to accelerate, and that is what they do best. If I need fuel economy, I'll drive the wife's 05 Mustang. For me, these cars are about smiles per gallon, not miles per gallon.
That said, I would be a big fan of overdrive transmissions, if I could afford them....
Just my opinion....
Importsmasher
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Scott Baggiore
66 GTO convertible 389/4 speed (parents bought new)
73 Firebird Formula 400/4 speed
74 GTO 455/4 speed
74 Grand Am
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