Depends what you want it for, I guess. I use 48x17 (74") for everything; it seems a good compromise. You can cruise at 30kph at a natural 85 cadence, spin out at about 50kph (around 140 cadence) and climb most things, albeit out of the saddle once it's above 10% (I did find the bridge ramps on the Lea Canal needed a run up or my legs would just stop, though). I use it for commuting, for RP laps (chatlaps and Z2/Z3 stuff - sub-7 on 74" is a bit frantic), for general getting around and for the occasional lower-paced Herts club ride if I feel like making my life difficult. I've tried 70" and just found it was too low a gear for general riding and hopeless for club use as even 40kph has you courier-spinning like a loon.
I'm on single-speed at the moment, but ran two brakes even when fixed - for safety and knee protection. If you're running fixed with no rear brake, you will need an uneven ratio - not just to save on tooth wear but also to give you multiple skid patches so your rear tyre doesn't wear too quickly.
I'm on single-speed at the moment, but ran two brakes even when fixed - for safety and knee protection. If you're running fixed with no rear brake, you will need an uneven ratio - not just to save on tooth wear but also to give you multiple skid patches so your rear tyre doesn't wear too quickly.