The stars lined up and averaged 20 MPH on today's ride (20 miles).
Didn't do anything different from any other day.
So, it's the weather (wind), rest (not riding over the weekend), bike (cleaned, tires pumped up and chain cleaned/lubed recently).
I know that the effort wasn't significantly different because of the heart rate data.
Only 6 minutes in "peak" zone & 42 minutes in "cardio" zone.
This is pretty typical data.
On high effort days, the peak zone can be nearly 30 minutes...