Will a 46/17 gear ratio on a fixie allow me to go fast? By fast i mean 20-25mph.?

In theory yes, with 700c wheels a 100RPM on the pedals should get you about 21mph. On the track the coaches always wanted you to spin @ 120RPM, which would be 26mph. This is too slow for the track if you intend to race, so 46x14 is better for the velodrome to give you a speed near 30mph.