Depends on what you are doing. I ride some trails, gravel pits, sand pits and roads. Right now i am at 13/40 because thats the length of chain i have :ermm: At this ratio im only running around 95km/h, sure its great for acceleration, but im in 6th wayyy to quick, and would like more top for the roads. So i'm going to go back up to 14/40 and see how i like it, if i want more top, then up to a 15 in front, if i can pull it :baseball_whistle:
Maybe this can help you figure out what you want:
Gear Ratio/Speed