Its a pretty simple math problem. Your goal time is a average speed of 26.1 mph. The 50x14 at 90 rpm is good for 25.8 mph, So cadence in the low 90's will get you to your goal, with the 50x14. Personally, I think the 53, and the 55 would give you too low of cadence, but you know best what your own personal most efficient cadence is.