Been doing NPC/Vista Ridge the past couple weeks in my 39x23. Speed on NPC is about 10-11.5 MPH (seated), dropping to 7.5MPH (seated) on Vista Ridge. I've been keeping my HR about 155 - 165 on NPC, and it gets up to about 175 on Vista Ridge.
I'm thinking should probably up the cadence with a 12-25 cluster... I don't like mashing on Vista Ridge.