Gearing depends on whether your using a fixed gear for conditioning or wrecking yourself.

68" is a long time favourite of mine, you can climb over anything and on the flat you will hold 27/30kmph at a cadence of 90-100, do the maths.

Low cadence training won't condition your muscle groups to their most efficient range (85-100) and when the pressures on you'll be jamming big gears to keep up.

Spin is king just ask Lance.