[brian015]

I'd start with the smallest pinion that would work with the 54t spur and see how it goes - you can always gear up if your temps are OK.

Here's what the calculator gave me for the 54t spur and the following pinions:
10T 36.22 mph
11T 39.84 mph
12T 43.47 mph
13T 47.09 mph
14T 50.71 mph
15T 54.33 mph
16T 57.95 mph
17T 61.58 mph