I went into this thread thinking in general terms— a cool day without wind, etc. For me personally, the optimum conditions for speed occurred on December 28, 2018. Winds were howling in from the ENE, so I headed almost directly WSW down the Santa Ana River Trail— pushed by a 25mph tailwind, with gusts into the 40s. Despite around 22 of the 70 total miles being on traffic-filled surface streets, I averaged 23.7mph on my final ride of that year’s Rapha Festive— meaning I had already ridden about 235 miles over the previous 4 days. Had it been my Festive opener, I think I could have managed 25mph.