I currently have a cervelo P2 size 54 and looking at a Speedmax 8 di2 CF. Here are my details. I am pretty sure they are right :-).
Height - 177cm
Inside leg - 80cm
Saddle height - 734
Y - 690
I currently use 165mm cranks.
Okay, let's talk. That Pad Y of 690... That's pretty high. I find this dubious. For a 54cm P2 to achieve that pad height there would need to be at least 168mm of "construction" between the top of the head tube (bike frame) and the arm pad. I'm trying to imagine how that might get divided logically: 40mm in headset top cap and spacer, a +6 stem is another 20mm up more, and then 100+mm of arm pad pedestal. The Pad X at 412 even seems short but not as outrageous as the 690 of Y - not after a fit. If you could remeasure while I do some math, let's meet back here to compare and discuss.
Based on your height, seat height, inseam - the other metrics you've given me... I would think a logical Pad X would be more like 460, and a reasonable Pad Y to be something like 620 - I'd even allow for 645, if you had something serious going on like 3 fused lumbar vertebrae.
If your numbers were in that ball park (I think they either are or should be barring any serious oddities) then it would be easy. The Canyon Speedmax CF 8.0 Di2 that's right for you would be a size Small. That bike comes stock with an 80mm stem that would work perfectly for you. The presumed Pad X gives you room fore and aft to make it perfect. Even the generous Pad Y I offered isn't the max on this bike.
Check it again and let me know.
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