48x18 vs 48x19

I run 48/18. I tried going one tooth down to 48/17, mainly for skid patches, but also to glide a bit more and spin a bit less on longer runs. What a change it made, for the worse. Although it gave me more top end speed, that was all I got. The bike went from quick, agile and responsive to a beast I had to man handle around. If I saw a gap in the traffic I couldn't spin up and be there in a blink, it was suddenly hard work.

I reckon 48/18 would feel very snappy for you coming off 15t. Fuji give you that default gearing - around 84gi - because they sell it as a track bike.

Going 48/19 for my own use would be too far in the other direction. If you want more skid patches you can do this:

• find your skid patches after a little wearing in (there will be three if running 48/18)
  
• Set one of them to be touching the ground while cranks are horizontal
  
• Remove chain from rear sprocket
  
• Rotate wheel so that the patch touching the ground is now halfway between two skid patches
  
• Make sure cranks are still horizontal
  
• Stick the chain back on rear sprocket.
  
  

Long answer, hope it helps - and that you chain is long enough even for the 18t.