Max tire pressure does not result in lower rolling resistance. Except maybe for an extremely smooth surface you wouldn't realistically be on. But in that instance there are no bumps to smooth out anyway.

Obviously an experiment is the way to know for sure. My guess is that it will be 'best' to use the same optimum tire pressure as you would with a normal road bike. It seems to me that a relatively small deforming of the right pressure tire is more efficient that having the tire,wheel and fork do the equivalent. I'm sure it's actually more subtle than that but my guess is the difference isn't big enough to make any difference in a real life situation. But I'm assuming the priority is rolling resistance first which may not be what you are asking.

They're not going to be direct equivalents, but I would imagine effective bike suspension will reduce the benefit of low tyre pressure. However, although suspension reduces the unsprung mass that relies on tyre deformation, there is still a need for the tyre to deform over surface imperfections and thus rolling resistance will not necessarily benefit from higher pressure (although I suspect the ideal pressure will be higher).

The best analogy here is low speed and high speed compression on a mountain bike fork. They are both accomplished by the shock but serve different purposes although the overlap means they do impact each other.

Tire pressure offers high speed compression while your isospeed decoupler/future shock etc offer you low speed compression. Getting the balance right is a art form which is why suspension tuning is a thing in the world of mountain bikes.

That is a good analogy. Everyone's advice and perspective has been helpful.

So for low level road buzz from pavement, tire pressure will do. For bigger bumps or holes, the suspension will kick in. Both help achieving the smoother, more productive ride.

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Indoor Triathlete - I thought I was right, until I realized I was wrong.

I have a 2017 Roubaix. It currently has 26mm Roubaix Pro tires on it (which measure almost 28 on the stock rim), and I'm running 80 lbs. in the front and 85 lbs. in the front. I train on roads that have their moments, and very few are "smooth" by anyone's standards. Here is my experience in what you are asking:

The flexi seat post and future shock front end aren't really noticeable when riding in most cases. Under high power seated efforts I can feel the seat move under me, but that is about it. I never notice future shock move unless I do it on purpose, like showing another rider that has never seen it. When I changed to this bike from my Allez there was a noticeable difference in "road buzz." Big bumps, expansion joints, etc. still rattled my teeth. The bike came with smaller width tires and the bike shop suggested 100 lbs. I weigh 180 lbs. by the way.

I slowly reduced the pressure on those ties and found that anything below 85 lbs. made the bike squirrely in corners. So I ran 90 lbs. and raced it all summer. When I replaced the tires with the current set I found that I could go down to about 75 lbs. before a tight high speed corner in a race scared me. At 80/85 lbs. that feeling is gone.

As for the big bumps, expansion joints and other things that used to rattle me teeth? Not so much with the bigger tires at lower pressure.

So in summary, the shock system takes out road buzz, the tires and pressure lesson the feel of the big problems. At least in my experience.

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"...the street finds its own uses for things"

Hmmm. What you described seems to be the opposite of the MTB analogy, if I understood that correctly.

Agree with you on tire pressure. 80 front tire and 90 rear tire is working pretty well for me.

Getting a 2013 Trek Domance 6.2 and still considering the new Roubaix as part of my age 65-70 plan. The more relaxed Trek geometry did influence the decision; yet, the Roubaix is supposed to be the better shock absorbing bike according to some reviews.

Thank you for your experience and pointing me back to my current tire pressures.

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Indoor Triathlete - I thought I was right, until I realized I was wrong.