The typical factory fixed gear comes with a 48t chain ring and a 16t cog, which, with 700X23 wheels and tires, corresponds to 78.8 gear inches
The phrase gear inches
refers to the amount of effort required to pedal a Penny Farthing Big Wheel bike having, in this case, a 78.8 inch diameter Big Wheel.
The original poster's 50 X 16 setup, with 700X23 tires and wheels, comes out to 82.1 gear inches.
I have ridden an 82 gear inch setup in an mixed urban/suburban/rural community having a dynamic gain and loss of about 1000' with success.
I can't brake going downhill by back pedaling at 82 gear inches.
I personally, in my riding area, consider 72 gear inches the most versatile gear set up for me and my riding area.
I can brake going downhill by back pedaling at 72 gear inches.
50 X 18 makes 73 gear inches.
I presently ride 42 X 19, which makes 58 gear inches, as part of a learning experiment.
Sheldon Brown maintains an online gear calculator.
Make sure to select the proper tire size, crank length (not so important), and gear inches
(as opposed to gain ratio
A more usable online gear calculator exists.
I got the link from a thread on this forum.
Anyway, I write from work and don't have it bookmarked on this computer.