Re: Rear Gear Recommendation
That tire is about 26" tall (pretty short). A 275 60 R15 is 28". I would get happy with the wheel/tire you want to run before picking gears.
With your tire, 80mph would be about 4000rpms with a 3.73 gear. This would drop about 500rpms with a 28" tall tire.
If this is a driver, top speed and cruising/highway rpm is important. I have 28" tall tires with a 3.55 TH400 and it seems like a good compromise given no overdrive. Overdrive seems to be a great option if you want to run more rear gear.
69 Chevelle Malibu
427 sbc, TH400, 3.55 12-bolt
n/a best: 11.24sec/118mph