On normal 40-50 mile rides with 2500-3500ft climbing I get 15-16 mph. On Sunday I did a 100 mile flat century and I really pushed myself to the limit and I got 18.6mph average.
After the first 50 miles my average was about 20.5. Then on the way back after taking a break my legs just died on me. I think my body ran out of energy(Didn't eat breakfast before riding)