Is the second decimal important in the ring/cog ratio?
48 x 17 = 2.82: 2.7% difference

50 x 18 = 2.78: 1.0% difference

49 x 18 = 2.72: -1.0% difference

I guess you might notice the almost 3% difference of the 48 x 17, but not the +/- 1%.