Setting up a single speed rain bike.. Using 42x18 right now (60.1 gear inches with 26x1.9 tires) good for acceleration but really have to spin it in the flats (and it's all flat around here) I tried 42x16 (67.6gi) which was much better for top speed but for "cruising" I felt like I was well out of my desired cadence and having to mash . I guess that's part of riding a SS.. basically feeling like you're in the wrong gear most of the time LOL. Gonna try to hit someplace in the middle and see how it works out, though I haven't actually tried riding in the rain yet.. usually I recall wanting a slightly lower gear than when running on dry streets. On my main ride I'm running a 46t front ring and I usually end up in 46x19 (62.3gi).
I'm not sure if it's because I'm riding 26" fat tire MTBs in city traffic but gear inches of 70-80 sound pretty nuts to me.