A larger gear in the front will make acceleration harder than a smaller gear. A normal gear (43/44/45-16, 42-15, 39-14, 36-13, 33-12, 30-11, 28-10, 25-9) works fine for just about everything but flatland (and that's more of a personal preference, I think - BMXTrix would know more about that than I). So, if you're looking for faster acceleration, you'd want a smaller gear up front to make the 13, like a 33.