90 is a pretty good cadence; I don't actually have a cadence computer or any other measurement stuff, but I put my gear ratios into Sheldon's calculator and it spit out my 90RPM MPH on each gear. It turns out I hover a little faster around 100RPM-ish when I can (and I double it for sprints).

I noticed there's hills I can get up at 15mph and hills I can't get up above 5mph; I also noticed that some of these hills are identical. So why can't I climb some of them?

Stop signs.

One of the long, not too steep hills I climb has a stop sign just above the base.

I don't have a low enough gear to get up in the RPM range, and standing exhausts me quick. It turns out that if I can get over 100 easy enough, though, the upshift to put me back around 90 isn't really bad... and all that happens is I wind up going to 100 again. The end result is I hit a hill at 8mph and wind up coming over the top at 15 or 18 no sweat; whereas if I hit it at 5mph, I'm still going 5mph or maybe 3mph at the top.

I still want lower gears...