OK I know there’s a 1500-post thread on MTB tires for gravel, and I admit I haven’t read it all. With that caveat, my specific quandary:
If I plan to run 2.1-2.25" MTB tires for gravel, does it make sense to try to optimize wheel aerodynamics? Zipp XPLR rims as well as recent clones from Light Bicycle and Nextie (image below) max out at ~40mm external, much narrower than these tires. I haven’t found tunnel tests of these tires with vs. without a deep rim behind them; given the lightbulb effect, do deep rims matter?
I know, horses for courses and narrower tires may be faster in some cases. But if we set 2.2 RaceKings as a given, is it worth the weight and somewhat harsher ride to get big ol deep/wide rims?
FWIW my A race is the Rift in Iceland, which seems to involve plenty of open/windy/flat-ish sections.
Lots of content out there from Enve, Zipp, Swiss Side, Hunt, Parcours, Rene Herse, youtubers, and others that conflict about gravel aero with extra fat tires.
I wish I knew the answer.
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Yes, I’ve seen much of that (I think), but most of the analyses I’e seen max out at 50mm tires, or compares across tires sizes but not +/- aero rims with a set (fat) tire. For example, 50mm is as wide as Zipp goes with their Goodyear tires approved for their XPLR rims.
Answering my own question, kind of, I went back and re-watched some of the DJ/Josh Poertner tests ahead of Unbound last year. I think this one has a good chat about this starting at 5 minutes:
I think the take home is that deep rims help some, though the system is probably not really optimal (rims are still narrower than they ideally would be.)
We’re totally gonna start seeing like 55-60mm wide gravel rims in model year ~2027 aren’t we? 
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Yeah this is something I was wondering; are rims just lagging behind tires in terms of width? Or at a certain point does the weight and stiffness of a ~55mm external/60mm deep rim make it impractical?
And obviously tires would need to be designed with less sidewall to work with a ~45mm inner dimension rim. Someone should just Grim Donut this and create such a setup, instead of approaching it asymptotically with 2mm wider rims each year for the next half decade.
And when will someone introduce a gravel tire with knobs/dimples that do double-duty on traction and aero? Seems like the RaceKings achieved that by accident, but I’m sure it could be optimized…
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Years ago there were deep rims that were normal aluminum rims with a non-structural carbon faring bonded on. The spokes passed through clearance holes in the carbon fairing, and if the bike was hung by the wheels, there was a risk of crushing the fairing. Maybe some version of this might make sense for big mtb tires.
Yeah I remember Mavic doing various versions of these. For a crazy deep/wide rim I wonder how the spokes entering the faring off-center would alter aerodynamics.
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