I checked the gear inch calculator:

[TABLE]
[TR]
[TD][/TD]
[TH="bgcolor: yellow"]58[/TH]
[/TR]
[TR]
[TH="bgcolor: yellow"]17[/TH]
[TH="bgcolor: white"]63.7[/TH]
[/TR]
[TR]
[TD="align: center"]13.3 %[/TD]
[/TR]
[TR]
[TH="bgcolor: yellow"]15[/TH]
[TH="bgcolor: white"]72.2[/TH]
[/TR]
[TR]
[TD="align: center"]7.1 %[/TD]
[/TR]
[TR]
[TH="bgcolor: yellow"]14[/TH]
[TH="bgcolor: white"]77.4[/TH]
[/TR]
[TR]
[TD="align: center"]7.7 %[/TD]
[/TR]
[TR]
[TH="bgcolor: yellow"]13[/TH]
[TH="bgcolor: white"]83.3[/TH]
[/TR]
[/TABLE]

starting from your 17T even with the 15T it's going to be the sweet spot for single speeds with minor hills and such. 14T is starting to push it in the 77GI's. But at 13T you are pretty much going to have a hard time to pedal anything but flat and slight declines, 83-84 gear inches isn't something you do on a hill climb. going from 17 to a 13 is quite a huge jump and I would be best if you try 2-3 teeth max.

Spinning out on a downhill you can always coast for all that matters, but having too high of a gear on a loong uphill and you are walking up the hill at 1/4 of the speed if at that since you'll be pushing the bike up too!