My wife recently got a Domane SLR6. . . Her 44 cm bike . . .

She's running 40c tires which run on the small side and measure out to 39mm and clearance is fine. But at least on her size frame I wouldn't run anything wider. I think 42mm wide which another poster mentioned is really pushing it unless there's more clearance on larger frames, you might get away with it on dry stuff but could definitely cause real problems in the mud.

My wife recently got a Domane SLR6. We recently did a 65 mile ride w/ about 70% gravel, including some pretty rough unmaintained Class IV roads and she did fine. The bike is very confidence inspiring, it rides really smooth and she's killing the downhills on it. If you do want to start doing pretty rough gravel or trails where you want 47-50mm tires then obviously you'd want a dedicated gravel bike. Also, it's not the lightest frame around. Her 44 cm bike is about 19 pounds with 32c GP5000 on some pretty light wheels, a pretty heavy Cobb saddle, Favero Assioma pedals, but otherwise stock Ultegra, and a bit under 21 pounds in gravel mode (wide tires and mtb pedals) with the stock wheels, which aren't the lightest. Obviously if you're willing to spend the bucks you can get it lighter but I don't think you'd ever get this bike down to 16 pounds without crazy exotic stuff.

She's running 40c tires which run on the small side and measure out to 39mm and clearance is fine. But at least on her size frame I wouldn't run anything wider. I think 42mm wide which another poster mentioned is really pushing it unless there's more clearance on larger frames, you might get away with it on dry stuff but could definitely cause real problems in the mud.

I recently got an Aspero and plan to use it for both road and gravel. As test I set it up for road and held my own in a fast group ride, even had a very good time for me on a short hill climb but don't currently have power on this bike so can't really compare with my aging but still light roadie. Swapping wheels is no big deal IMHO. However, the compromise comes in two other areas. I am using SPD mountain bike pedals for gravel and really wouldn't want to use those for fast road rides, and adding a pedal swap on top of the wheel swap starts to get into PITA territory. The other compromise is in handlebar shape and position. For gravel you generally want to ride a higher handlebar position so for road riding I have to bend my arms more to keep from being too upright. The Aspero comes with flared bars and I find that wider drop position is awkward on the road. But lots of people don't use flared bars for gravel so if you are primarily doing road then I think you could set up your bars for the road riding and be OK.

So just some stuff to think about.

That was me. My wife's Domane SL5 is a size 54, and I'm pretty sure it would hold 45's. Thanks for sharing your experience. The Domane is a great all around bike, with the only downsides being weight, as you mentioned, and it isn't a very agile bike. But that stability is much appreciated on gravel.

The larger frames must have more clearance, then. The tight spot on hers is the seat stays, which of course are very low on such a small frame so not as much room to bow out. The other areas of the frame appear to have more clearance, particularly the fork.

My question is: Since I ride mostly road, if I'm running the same 28mm tire on the same rim on a bike marketed as endurance road like the Specialized Rubaix and a bike marketed as Gravel Plus like the 3T Exploro how much of a performance loss am I looking at on the Gravel Plus riding on pavement? Is this performance loss attributed to my power output/aero loss due to my position on the bike or the bike's performance?

Between these options will the Caledonia severely outperform the Exploro on road with the same tires?

Budget is in the $3k's but that being said the Ribble CGR is tempting me with their value at the 105 version.

Thanks in advance!

Check out rondo (https://rondo.cc), they have an interesting fork feature that allows you to change the geometry subtly. I don't know anything else about them (although their models get good reviews on bikeradar), but my LBS has begun to stock them.