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...

I find I get up hills faster by powering up at lower RPMs, like 80.

My 90RPM speed is 6.8mph* in first, and I tend toward 8.5 comfortably. I have trouble getting above 5mph on some of these hills, probably because I'm out of shape trying to do a 7.7-8.5 mile commute (depending on diversion taken: the longer one goes through the mountains and involves a long, heart-shattering hill; but it avoids an annoyingly narrow low-traffic road that I get passed dangerously on) through ungodly hills.

Apparently I'm coming up those hills around 60-70 (4.5-5.25... not even, I barely break 5.0) at best, and laboring around 3mph at the top (like 40). Hmm according to this, my nominal cadence is what ... 105-108? And around 180 in sprints... there's no way, I need a computer with a cadence meter. :| 180 isn't physically possible, I'd never be able to shift without running the chain right off!

* 30-42-52 crank with a 13-32 8 speed cassette and 700x32 wheels. https://sheldonbrown.com/gears/

I have found that too. But I also found that for the next five minutes I go a lot faster if I went a little slower up the hill at 100 RPM.

Really? Could something else be going on to explain your speed? I've seen as high as the 180s on my cadence sensors while I'm doing spin drills ... but my sprints tend to involve hammering in lower gears, most of the time.

Keep practicing. I find that hills never really seem to get easier, but eventually, as your cardiovascular system improves and your legs get stronger, you will get up them faster.

I can generally recover in less than a minute, and I'm still way farther ahead than anyone I passed along the way who was spinning in low gear. Not that it's a race, I'm just very destination oriented, and the quicker I get there the better (for me).

Sprinting for me usually involves "I'm going too slow and the terrain just got easier and I'm not exhausted... *pedal fast!*" and then I shift up 3 gears real quick and start to slow my pedaling down. The longest I've gone is like 15 seconds, through 5 gears by the end (after the initial burst and upshift, I slow down significantly, but not to my normal cruising cadence), and then coast for several second... :/ It's like my body just shuts down; I curl up against the handlebars and pretty much don't move for 5-10 seconds unless there's suddenly a hazard, I feel like my muscles won't really respond anyway. Even muscles I didn't even use. Kinda just want to nap on the bike o_o

That's usually me being silly thinking I can go from 10mph up to 25mph immediately--on my second chain ring, on my second largest gear in the back. While that technically happens, the after effect is that I'm completely broken for a short bit.

They seem to get easier for me... but it seems to be (ironically) easier to go up them faster.

Kevin Mansker at 257 rpm, interval

Ahhhh. I wouldn't call that a sprint, personally. In fact, it kind of sounds like the high-cadence drills I do, to practice spinning the cranks really fast.

Like you, I find that hills get easier. Kind of depends how you mean that, though. If you go out and spend an hour climbing hills, it won't be easier; being stronger will mean you do more climbing. On the other hand, if your plan is to get to work, then yeah, the hills along the way get easier. It's like lifting weights at the gym, and not adding more as you get stronger.

The thing that spinning a lower gear gets you is efficiency. When you spin, you use mainly slow twitch muscles which are capable of producing constant effort over long periods of time. When you mash, you use fast twitch muscles which are capable of much higher bursts of power but exhaust themselves very quickly.

That's the thing that makes stop signs on hills so evil. You use your fast twitch muscles to get the bike started, and there's just not enough in the tank to keep accelerating. Keep working on it, and you'll get better but the problem doesn't really go away until you get someone to remove the stop sign for you.

On my daily commute home, I've got a traffic signal that is almost always red when I get to it followed by a half mile at 3-6%, which is just enough to tap out my power reserves. Then, I have to come to a full stop and make a left turn across traffic before climbing the really nasty hill up to my house, which goes 10-20% for a quarter mile. I can cruise up the hill after the red light at a pretty good clip, but for the final climb I'm lucky to get over 40 rpm on my regular commuter with its 36-27 low gear. When we first moved to this house I joked to my wife that I was being passed by pedestrians on my way up the hill. Now (a year later) it's starting to get a little easier, but I expect it will always be a workout.

FWIW, I did ride this climb recently on my mountain bike with a 22-32 gear, and I was able to spin like mad with that but the aerobic effort of spinning didn't combine well with the general oxygen requirements that a steep climb has to begin with. That's probably why people above have mentioned 80 rpm as the sweet spot for climbing.

I can't imagine many situations on pavement where I'd want a gear lower than 30-32. Stick with what you have and I suspect you'll be happy with it soon. Think about the effort you're putting in just before the hills that give you trouble. If there's any way you can conserve energy before the hill, do that. I used to hammer down the descents on rolling hills, thinking the extra momentum would help get me over the other side, but I've found that it's much better to leave the acceleration to gravity and save my legs for the climb.

If you have some spare time, you might want to do a few hill repeats.

Mandatory Greg LeMond quote: "It doesn't get easier, you just get faster."