Ditto Ceya. I ride low gears on the street (I like to spin) which are 49x18 and 48x18.

For track, I've been riding 50x15 -- there's no way that I'd ride that in the street. That's bad news. I think 48x16, the typical gear that most off-the-shelf track bikes come with is a tad high too -- especially if you're starting out. But I guess, they're supposed to be "track bikes" anyway.