jax, I'm not sure what exactly you are asking for, but basically there are a few things you can use to help you figure things out.

1. What do gear combos mean? Gear ratios determine how many rotations of the wheel will occur with a single rotation of the cranks. The crank numbers (39/52) divided by the cluster number (12-23 or whatever) gives you this number. So, if you are in 52-15, the rear wheel will rotate 3.46 times per revolution of the crank.

2. Based on the information in #1, figure out what cadence (number of crank rotations per minute) you like to ride (typically 80-100), and at what speed you typically travel. Use the circumference of the wheel to back into the rotation you desire. Assuming you have a 700-23c wheel, you can guestimate the circumference as 210 cm, or 82.7 in. If you typically ride 18 mph, that means 229.91 wheel rotations per minute. Divide this by your desired cadence (say 85) and you find you need a gear ratio of about 2.7. This should be about mid-cassette. If you are in your 53, that would put you in the 19 or 21 cog, and in the 39 it would be the 14 or 15 cog (but you didn't mention a 14, and I'm assuming you are running 8-sp)

3. By my math, assuming an 85 cadence gives you the following speeds with each of your gears:

Code:

39 53
13 20.0 27.1
15 17.3 23.5
16 16.2 22.0
17 15.3 20.7
19 13.7 18.6
21 12.4 16.8
23 11.3 15.3
25 10.4 14.1

Now, maybe with this as a basis, you can explain what you are trying to do...#1: shorten the jumps between gear spacing but keep the overall spread, #2, shorten the the total width and shorten the spacing, or #3 I need more high speed gears, or #4 fewer lowspeed gears.

Unless you are wicked fast (I'm not discounting this possibility) it seems like you have plenty of gears for typical riding. If you are racing I'd say go for an 11-21 cassette, but for the most part, I'm not sure what your goals are, so I'm having trouble assessing the best information to provide.

EDIT: Just saw you have a 9-sp. Assuming its a 14, your new numbers are 18.5 and 25.2.