chainring clarity please...
 

Ok, I am running 48/16...the area I ride is mostly flat with a few hills...when I battle the hills I feel like I'm mashing down on the pedals and I end up feeling it in my knees later. I would like a lower gear to pedal faster. My question is this...How much of a difference will I feel if I switch to a 46T...a 44T...a 42T?...I think you now where I am going. Thanks for being patient with a nooby question...

I was having the same problem too, mashing up hills paying for it in the knees, so I jumped to a lower gear ratio. Since cogs are much cheaper and less labor intensive to switch out than the crankset, I changed out my 16t cog for a Surly 19t cog. After switching it, if you use the same chain, you have to move the rear wheel closer to compensate, and you could be in danger of having your rear wheel rub against the frame. Luckily, i have about 1-2mm of clearance and everything is fine and dandy.

I would advise swapping out the rear cog rather than the chainring, if only because it's more cost effective. To get a better idea of what you're pushing, check out this link: http://www.bikeforums.net/showthread.php?t=468351

So you're pushing 81 gear inches right now, which is pretty high for the street. I'm running 71.5 gear inches at 45x17, but you could achieve roughly the same results with an 18t rear cog(72gi), or even run a bit lower with a 19t cog (68.2gi).

If you don't skid, ignore this, but taking a look at the second image, a 19t cog would be a good idea because it would give you 19 skid patches, as oppose to only 3 with an 18t. 17t would be good too, but 76gi might be too high.

It's impossible to tell you how much you'll feel the difference - you'll understand when you experience a variety of gearings. Once you know how the difference between a 70 and a 78 inch gear feels, you'll be able to answer your question yourself.

I suggest you get a 17t cog. The lower the gear, the better a cyclist it will make you - forcing you to develop a smooth, fast cadence that will be of good use to you when you push a larger gear (for racing) or when you get a road bike. My "winter training" gear is 46x18.

i've ridden 42/16 (~70GI), 46/16 (~75GI), and 48/16 (~80GI) and the differences between each of those are definitely noticeable. personally i find 48/16 too high for general street riding. i like 42/16 for commuting and casual rides but i can't really sustain 20+ mph on it for very long and i max out at like 26 mph so i think it's too slow for faster group rides. 46/16 is a good compromise between speed and comfort for the street but that's because LA is relatively flat in most areas.

Thanks for the replies. I like the idea of changing out the rear cog...the whole money thing. I'll try it and let you know what I find...thanks again.

I agree with the changing the cog unless it is a kilo TT (I'm making the assumption based on the 48/16 ratio) and comes with a Dura Ace cog, then I'd put a salsa ring up front and have a better quality ring than the stock truvativ one for not too much more money than a quality cog.

81 gi is way to high in my opinion -- you will ruin your knees and you're not getting the type of enjoyment you should out of your bike.

My advice would be to get a new cog that puts you in the 67-71 gi range -- not only will this help your knees but it will help you build endurance (you need more rotations) and your pedal work will benefit from the need to slow down/speed up. Good all around to change it up, IMO.

Thanks for the help everyone. I put a 19T cog on the back which puts me at 68 GI...what a difference it makes after a week of riding it. My cadence is way up, I can charge up hills and most importantly, my knees aren't hurting after a long ride. Thanks again.