Go here:
http://sheldonbrown.com/gears/
With a 48t chainring, going from a 16t cog to an 18t cog will make an 8.7 gear inch difference.
A big difference.
From 78.8" to 70.1".
For small differences, change the chain ring instead of the cog.
For big differences, change the cog.
It cost about the same.
By the way, gear inches have almost nothing to do with sore knees, other than revealing inelegant body mechanics.
If a person does not want to put the work into changing how they pedal/spin, then, by all means go to lower gear inches.
I personally find 70"-72" very pleasant and versatile.
Buy a 43t chainring from Harris Cyclery and that will give you 70.6 gear inches.
I like a 47t chainring and a 17t EAI cog for 72.7 gear inches.
If one puts the Sugino/SR 130 BCD 47t chainring on the outside position of the crank star, and puts a 17t EAI cog in back with the flange towards the hub, it will give him or her a perfect chainline on a Pista.
I mean perfect.
Putting the chainring on the outer position will require spacers, so ask Harris Cyclery for some of those if you order the Sugino/SR chainring.
I think the chainring, cog and spacers comes to about $70, total.
At 59 years of age, I ride a 52t chainring and a 17t EAI cog for 80.4 gear inches, and I don't have sore knees.
However, I study body mechanics incessantly.
If a person doesn't want to do the body mechanics thing, go down to 70"-72".
When and if you use Sheldon Brown's Gear Calculator, remember to select your tire size and gear inches in the respective fields.
I have four chainrings and four cogs, and I experiment with them.
Fun.
I highly recommend EAI cogs.
The different cog manufacturers have slightly different threading (in my humble opinion) and switching back and forth between different manufacturers damages the hub threads.
Or so I think.
