I started with a 17t. On our outdoor track it wasn't quite enough down the straight, so I went to 18t without issue. I've contemplated trying a 19t a couple times, but haven't done it yet. If you have a very large track with a long straight, expect to try the 19t. If you have a small technical track or not much of a straight, expect to be in the 16t or 17t range with the 1900.