With my TKO600 (.68OD), my GTO initially had a 3.55 rear gear. Cruising around my area (country 2 lane roads) I felt like I was either running more rpm in 4th than I liked or bumping around in 5th gear at lower rpm than I liked. I swapped the 3.55 8.2" for a 3.73 8.5" and it seems a better fit. Driving style and usage did make a difference for me.
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frittering and wasting the hours in an off hand way....
1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. 
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