20 If you are lucky. I run a 48 tooth and my top speed on flat ground is 28mph. I need that bigger sprocket though, I live in an area with a lot of hills and my house is on top of a great big one. Honestly, 28 is fast enough for me though.
We'll get a real test here sometime next week, I am building a hill climber. A Schwinn Del Mar with a 56 tooth sprocket. It's for my Dad, he wants that huge sprocket so that he can haul heavy loads of groceries up the big hill. The 415 chain he ordered just came in today, then I just need to put together a long exhaust pipe like he wants and we'll be in business. I'm just waiting for my next day off from work now.