I've run 3.91, 3.55 and 2.76 gears in my current car with basically the same motor (cam change). I ran it for years with 3.55's and it was a good "all around" gear. In my opinion, not much different than 3.23 gears. I was running an old purple shaft 292/509 cam and the car loved the 3.91 gears for "spirited driving around town" and would bark the tires in 1st/2nd/3rd. HOWEVER, without an overdrive, it sounded like it wanted to spit a rod out the side of the block on the highway it was revved so high....3200 rpm at 70mph (roughly). I wanted to drive the car cross-country & I didn't want to spend $5k-$6k on an overdrive, so I put a 2.76 posi centersection in the rear end, tallest rear tires I could fit, and a slightly smaller comp cam....much less off the line acceleration, but it cruises at 2300 rpm (roughly) at 70mph and I can drive for hours and hours with no noticeable issue. My mileage went from 10mpg highway to 14mpg highway