Ok, so I have finally decided to plunge into the single speed thing and I am converting an early 90's Trek 400 road frame into a single speed roadie. I considered going fixed but the hills in my area can get pretty crazy and I want to be able to coast when I need to. So, from what I have read, I decided to go with a gearing that will keep me working, not too easy, and will still be able to keep up with some geared friends when I get my leg rpm's up. My question is, how do you all find the way to spin easy enough for your recovery days? Do you keep a lowered gear wheel set up for those times? Do you just decrease your leg rpm using the same bigger gear to lower your heart rate? Thanks for the help.

You need to ride fixed. If you're going to go through the trouble of converting an old road bike, you need to at least make it worth your while. If you can't stand it (won't be the case..), you can just thread on a freewheel.

If you get a flip-flop hub, you can keep a smaller cog (or freewheel) on the other side and flip the wheel if you need to "recover." I admittedly don't know anything about proper road bike riding technique and have no idea what a "recovery day" is. I just ride my bike. If I'm tired, I ride less or take it easy. I don't have any problems.

If you're using a single speed for training, you'll want to keep the ratio lower, i.e. 46 x 17 or lower. Generally somewhere around 70 gear inches or lower. Thing about training on a single-speed is that it should be exclusively about your spin. Single-speed mountain and cyclocross racers are generally "churning butter" at race time, so that on more difficult sections they don't have to mash. Now if you're using the bike for general transportation, or work, then you'd want to have a higher gearing, based on topography. Working in NY I rock a 51x18, but when I lived in Portland, I ran a 51x20, which is 68 gear inches. Pretty freekin' spinny.

but to actually answer your question... I don't have recovery days.

+1 for a rear wheel with a flip-flop hub

But when you try fixed you're not going to want to go back to using a freewheel I guarantee it, fixed is just way too fun and it makes you a better rider in all other applications.

Riding fixed makes your d!ck bigger, too. And you'll fart air freshener as well.

On your recovery days, flip the wheel over (you will have a flipflop hub, right?) for an easier ratio, or ride a gearie.

LOL!!! The area I live in is very, very hilly. It seems that I am either going up or down, no flats. When riding my geared road bike and testing to see what gear will work best for me for a single speed, I seem to need a 42 x 21 to climb these hills and if I went one gear higher I would probably have troubles. I also don't want to introduce any knee pain. The thing with that gearing is that on the little flats that we do have I seem to be spinning like crazy. Perhaps I should just except the smaller gear and work on my spin for the flats? Is a lower gear higher rpm better on the knees that a larger gear with lower rpm? Again, I feel that a fixed would cause me problems on the downhills because of the smaller gearing. I don't think that I would be able to maintain a 160+ leg rpm Thanks for all the advice.

yes a lower gear is better. the spinning is less stressful on your knees than the slow hard strain on making that revolution on higher gears.
If you're worried about hills, slap a brake on there.
Also don't believe these nuts who say "once you go fixed, you never go back". It's true for some people but riding a fixed bike doesn't spoil all your other geared/ss rides.

compromise is key, and bmx cogs are cheap, so you can try different ratios until you get something you like. for a very, very, hilly area it seems like you're destined to do some spinning, which is not necessarily a bad thing. i'm no expert at any facet of cycling, but the hardcore roadies at the local bike shop are always talking about raising their maximum cadence and sustaining it for longer.

and without directly contradicting those who know a lot more than i do, i can say from my experience the lower the gear, the happier the knees. i find that using a tall gear on a single speed (freewheel) bike does much less damage than using the same tall gear on a fixie. most of the knee strain (on a fix) seems to happen when i'm slowing down. hope that helps. good luck!

agreed, i still love my gears. riding a fix just helps me spin them faster.

remember lots of the advice you get here are from people who have no concept what a hill really is...

for my moderately hilly commute (one steep hill on the way home) i run a 44x18 and seem to do ok! but when i get out of my element into the bigger climbs i just pull over and walk them when i can't pedal anymore.

singlepseeding will make you a better masher. so after running the 21 on the back for a while you might be able to slowly start going bigger. or you could start a little higher and resign yourself to walk a little till you get stronger.