I just went on a decent ride with my junk (huffy) 27" roadbike and left it in 55/19.
I found the ratio to be easy enough, in the flat, if too comfortable. However, there arent any hills in town to try....

The Nishiki has a 51 & 48 chainring set
I have a 19t cog laying around.... So I wil compare them.

55/19 = 2.89 : 1
51/19 = 2.64 : 1
48/19 = 2.52 : 1

I'll test the other 2 combinations and share the results....