someone once told me for a fast and dry course 63gi is the standard. adjust accordingly to conditions/fitness.

I have never 'really' raced but I do a local ~70km mixed terrain point to point called paris2ancaster. the past three years I have used 42/16(~71gi) and had to run all of the real sloppy stuff. figure I made it up on the rail trail and small road sections with the ability to go faster. that's what I tell myself anyways.