hi, first off, please forgive any ignorance or stupidity... i am indeed kind of green when it comes to bike knowledge, but i'm attempting to get better....

OK
i have a redline 925 and am running the stock gearing of 42 X 16

can somebody recommend perhaps the 'next logical step' in gearing for getting more speed out of the bike, but not necessarily something that is going to give me a stroke going up hills? i'm pretty much OK on hills with this gearing (depending on the day and my mood!) but on the flats i'd like to average more like 25mph than 18-22mph.

43x16 would be the next step.
42x15 is probably your best choice

which is faster, 43x16 or 42x15? how do i learn how to calculate these things so i can stop bothering you kind folks with my ignorance?

43/16=2.6875
42/15=2.8

that is the number of revolutions your wheel will make for each revolution of the cranks.

www.sheldonbrown.com/gears

for a 700c wheel, gaining 3 teeth in the chainring is about equal to dropping a tooth on the cog. that's how i think of it.

That is ONLY true at around a 3:1 ratio. ie 48x16 or 42x14.

drop a tooth in back...cogs are cheaper.

well i'll be verschnackled. i never realized that.

so should I change the back cog to a 14 instead of a 15 ?

you could if you really want to step it up. I would try 15 first for a while if it was me

it's important to keep in mind that as you drop teeth off the cog it'll be a bit harder to get up hills and make quick accelerations. going from sixteen teeth to fourteen might take a little getting used to.

fourteen is a pretty small cog, i'd maybe go with the fifteen but it's all your call.

i'd drop to 15. a two tooth difference in a cog is fairly significant. why don't you get both to feel what it is like? cogs aren't $$$ - about $20 - so it's not like buying a couple of chainrings (for the record, rocket rings are $20, fit 110 and 130 bcd, so if you ever need/want to change your chainring...).

i started on 39/14 and went to 46/16. not a huge change, but i felt it.

do i need to change my chain when I go from 16 to 15 or 14 in back?

absolute cog size is meaningless. 11 is a tiny cog but 26x11 is still a really low gearing. 42x14 is the same as 48x16 which isn't an unreasonable gear.

No, If you have enough room in the trackends you should be able to just put the wheel a bit farther back. Worse case you will have to shorten the chain by a link. Going in the other direction could require a new chain though.

anybody have any recommendations on a brand of 14 or 15 cog?

dura-ace.

Your 925 has a 15 tooth cog on the fixed side of the flip-flop rear hub.

Get up 5 minutes earlier.......

i still have the scars from the last time i rode fixie.... i will keep trying, but my default/safety for getting from point A to point B is the SS freewheel.

Spin faster.

ah.

freewheels, with few pricier exceptions, don't go beneath 16teeth. you're probably going to need to up a few teeth on your chainring. i suggest a rocket ring (available from harris cyclery - sheldonbrown.com), which is decent, cheap, and fairly tough. oh, and it fits 110bcd and 130bcd.

dicta makes one it doesn't get much cheaper then that.

Dicta makes a 15t freewheel that fits standard threading? This is news.

My idea was to try the 15 tooth cog already on your bike to determine if it will give you the speed you want. I'd find a straight, quiet, flat road, like the access road behind a big shopping center, and crank it up to full speed. Then find a steeper-than-average hill in a quiet neighborhood. You might find that the stock SS setup on your 925 is the best compromise. I took a 50 mile ride on this bike last week, with a fair number of hills to climb. It was a good setup for that.