I've decided to build up a fixie and I'm trying to figure out gearing. My main question is how hard is it trying to control your speed down or how easy it is to kinda just let your feet kind of float in place on the pedals while spinning. My big problem is that I have some rather large hills that I'd rather have low gearing to climb, but I'm not sure if I should go with the highest gearing I can to be safer on the down hills.

P.S. When commuting on my mountain bike I can regularly get up to speeds of ~30 mph / 48 kmh

I just started riding fixed two weeks ago, a Trek 330 conversion at 44/15 gearing. Not too many hills around here but when I do have to climb one I find a fixed gear to be easier going up (probably due to the lack of coasting, the low pedal carries itself back around to the top due to the wheel's rotation, making for an easier climb); going the way down can be a little scary at first. I guess you can either go at it and pedal hard down, or try to just let the bike do its thing by "ghosting" your legs on the pedals, just letting the pedals do all the work. I wouldn't recommend that though, it's probably better to be in control of how fast you're going. I usually use a bit of resistance to keep my downhill speed low, so that I don't have to pull hard to stop at a red light or stop sign, etc. It's a great workout, though.

Run a front brake and use it whenever you need to keep your legs below a certain speed. Letting the bike get away from you without any fore/back pressure can be pretty wild at times - don't blow out a knee. I've come close to spinning out a bunch of times in Toronto already (my max seems to be around 33mph/55kmh with 46/17) and it's not so much fun after a few seconds

Having a brake is worth it for being able to modulate/hold downhill speed steady.

+ 1 Definitely use brakes for hills.

Start here:

https://software.bareknucklebrigade.com/

under 70" is good for hills and a good gear range in general.

Also, taking your feet off the pedal seems like fun at first. And then you realize you need to get your feet back on the pedals that are spinning at like 300rpm to stop.

i get into some pretty gnarly hills up here, and I MUCH prefer going up them then down them. I'm riding 46x16, which I think is pretty decent. I can make it up some pretty decent stuff by just standing up and digging in for a bit, and downhill I get some decent speed before the bike outspins me and I have to hit the brake to keep her steady. If you're going to be dealing with hills that outspin you frequently you're going to want a brake.

*edit

Also, to answer the OPs question, you can't really just let your feet sit on the pedals, otherwise the pedals will pull your legs which will start you swaying from side to side. You have to keep up with the bike if you want to keep it steady.

get a singlespeed.

42x15 is good. (28" wheel)

Yeah its faster.

the only way to roll is with a front brake i'm afraid. making it easy to climb up makes it that much harder to go down, as others have mentioned. there is nothing wrong with having a front brake. call it an emergency brake if it makes you feel more comfortable. i always thought the upper peni was pretty flat, but most people think milwaukee is flat too... hills are hills san fran or omaha be safe!

I've had some good luck with slaloming down the hill and moderating your speed with skids, but if it's real bad, just throw a brake on.

That's a good way to practice a crappy descending technique.

That's downhill, right? Otherwise you're TDF material.

I'd recommend 15/42 or 16/44 for the gearing, depending on which parts come around.
If you have troubles with the hills at first, just stand up and if necessary kinda s-shape it up the hill.
For down hill, I would definitely recommend the front brake until you get used to riding fix.

actually, I find that it is much easier to climb hills on a fixie. A fixie forces you to continue pedaling, which forces you to maintain momentum. Even a momentary loss of momentum can cause you to stall when climbing a big hill.

as for gearing, go higher than you think you should. You'll have sore legs for a few days from climbing up hills, but the high gear will pay off when you aren't forced to pedal at 300 rpm down a hill.

I also suggest wide bullhorns so you can really crank your way up the hill.

Great advice if you want to blow your knees out in about 2 years. It's not the spinning down the hill the other side, it's the mashing at a "high gear" up the hill beforehand that does it.

100% bad advice.

I imagine it probably works both ways, spinning 300rpm can't be good for you either. It's more about finding a balance, which is definitely not big-gear mashing up hills.

70 gear inches for the mediocre suburban hills here. almost all declines have an intersection at the valley and an immediate rise back up, so the general technique is to spin down the hills, yell like a wild baboon and pray drivers stop at the stop sign, and get your ass up the next hill.

Why can't you unclip and use the front brake?

That's what I'd do...

"That's a great way" to shoot down other's advice without giving good advice of your own. waste of space.

im not so sure, If you have a good fit and are getting decent extension then climbing up in a bigger gear isn't too hard on the knees. Who knows, maybe my knees will be wrecked in another year. I'll keep you posted.

with technology today, you can just get a new knee.

I'm running a fixed at 44/17 and I have both front and rear brakes. I generally don't touch the brakes except on steep downhills at which point I use both... I like the balanced feel of braking front & rear. I really don't see much advantage to not having both brakes on the bike... they don't weigh much.

The bike looks better with less brakes. Oh, and I only have one lever with no matching pair.

to the OT: its probably a question you'll answer yourself once you start riding.

but go with a lower gear for messing with hills, going down a hill in a lower gear isnt a big deal - you'll learn to control yourself. but you will really appriciate it when you're going up the hill and not killing your legs.