Yet another data point. Today I took the P2 out on the same route as above. The results were much more comparable to the Shiv than previous rides. My average today was 18.9mph over the same route. That might seem like a dead heat but conditions were a factor today that weren't a factor the other day. In particular, I started out with noticeable wind that increased quickly over the short duration of the ride. In my previous ride, I had almost no discernible wind to deal with, probably <10mph. However today I started out the ride in 10-15mph wind that only increased as I was out. In the short 75 minutes it took to complete the ride, my seat of the pants estimation is wind had picked up to a steady 15-20mph with the occasional gust of 25mph.

Not really sure how much of an affect it had overall as the ride is something of a loop, so a headwind in part of the ride is a tail wind elsewhere. I will say in one particular stretch of relatively flat road that's around 3 miles I was comfortably riding at 25+mph with a tailwind. Now I can often sprint to 25mph in short bursts, but to be able to hold that pace, much less comfortably, that's pretty significant.

I still have a goal of getting the Shiv out and back on that 37 mile route without incident for a good data point, but as of yet, the stars have not aligned and the cycling gods have not smiled on me.

I definitely think seating position is playing a factor in the difference. As such, I did an internet search and there is a Shiv seat post that positions the saddle even farther forward than the standard post allows. I think I'm going to track one down and give it a try. I should be able to pick it up for <$100, and if that's the case, then I should notice some parity between the two frames.