haha.
I test rode both the Tarmac Elite ($2,000, 6r carbon) and the Tarmac Expert ($3,000, 8r carbon) last Tuesday. I personally filled the tires to 100psi on each of the bikes so I was sure that they had the same tire pressure. I also checked the actual tire - they had the exact same tire, but different wheels. As I mentioned before, I had been fit to both bikes by the same guy, and they were both the same frame size.
I took out the $2,000 Tarmac for a half hour ride first. I rode on sidewalks. I deliberately hit bumps. I didn't wear gloves.. I even rode it about half a mile on the crushed limestone trail by the bike shop (would have liked to have gone farther, but I ran out of time). For the 1st 10 minutes the ride was fine. As I got near the limestone trail, hitting bumps started to hurt a tiny bit. When I hit the limestone trail, riding was...unpleasant. By the time I turned around and got off the trail, any time I hit anything (railroad tracks, curb bumps, whatever) it made my wrist hurt with a noticeable, sharp pain. It wasn't "really painful" or anything, but I would definitely describe it as being just past the thresshold of "annoying" and clearly into "painful".
When I got back, I inflated the tires and took out the $3,000 Tarmac (I keep referring it by price because the 1st time writing this I accidentally wrote Expert rather than Elite a couple times - don't want to get them confused). I took the same route I took with the first bike. I'd really wanted to like the $2,000 bike better - 1 it's cheaper, and 2 I think it looks way snazzier. I did the same thing - I rode it on the sidewalk, hit some potholes with it, and took it out on the crushed limestone trail. But even though I started out with my wrists still a little sore from my previous ride, this bike just clearly had a more comfortable ride. There were no "small" bumps that gave my wrists a jarring feeling like on my previous ride with the other bike. I could still feel bumps, but they were just more...muted. By the time I got to the limestone trail, my wrists were still fine. I hit the limestone trail, and it was good. I wouldn't call it a dream ride or anything, but whereas on my previous ride it hurt, now it would just "slightly annoying". I could see how it would probably be fatiguing if I rode the trail for hours on end, though I wasn't able to test that out. On my way back, the ride was still great. I hit railroad tracks (the ones that are sunken into the pavement), those little dips in the curbs, I sped along a bumpy sidewalk - still all good.
(I would add in the caveat that it wasn't the totally smooth ride that I remembered the $3,000 Roubaix having - the $3,000 Tarmac did give me that annoying/painful "jolt" that I disliked whenever I plowed across a real pothole, like one of those foot across ones. But the Tarmac is more fun to ride, and I felt like it accelerated a little faster.

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So that's my experience! I definitely felt that the 8r carbon $3,000 Tarmac had a far more pleasant ride than the $2,000 6r carbon Tarmac. In my opinion, I've ruled out tire pressure and riding position as factors, leaving only frame construction and the wheelset as possibilities. Let me know what you think, and what your experiences have been.