Dylan Johnson has done this, and his conclusion is that wider tires are faster for gravel. He has reported measured data on the aero penalty, but has not yet done the same for rolling resistance. Other than share his overall conclusion that I mention above.
I got this in my inbox a few days ago. Maybe this will talk more about aero vs other tradeoffs.
The problem with Dylan’s testing it that it only pertains to those sections tested….which seem like they were always the chunkier, slower sections.
Do those same conclusions hold for the whole course? That is the central question that I believe still remains unanswered.
What does he recommend now?
I think the most important thing, beyond the testing, is to ride the tire that is fastest for YOU. If you are more confident on a certain tire, run it!
Although not specific to gravel tires, this is some more data on the same topic of impedance.
This is the info I’ve been looking for. Thanks! 
He has said verbally (IIRC in the podcast below) that wider tires are better for all but the smoothest sections - but has not shared any test data to back that up.
As of 10 months ago (also in the above podcast) he was still recommending them.
With his new gravel bike with bigger tire clearance, he may now be recommeding even wider tires 
I’m going to go with the (controversial?) take that Dylan Johnson is not the Messiah he’s made out to be and cherry picks his research to generate clicks
He’s not infallible, he spends a lot of time experimenting and questioning the status quo with his own ideas to find the fastest set up. I think he is probably part of what has driven the pursuit of marginal gains in gravel, he didn’t any of them but certainly contributed to the popularization of them.
He’s running Continental Race King MTB tires. 2.2" I think.
I thought he was running Thunderburt 2.1 as the latest revelation.
I think he tried them and showed the new Felt with the Thunderburts but when he actually raced I think he was back on the Race Kings, not sure why.
I don’t see what’s wrong with 47mm pathfinders.
It’s a durable tire.
I saw Dylan was trying out the 50mm Conti Terra Hardpacks but he hasn’t mentioned what he thinks of them yet. I mentioned them to him in the spring last year but he couldn’t fit them on his Factor.
I used them all last year and they “feel” a bit faster than the Pathfinders I was using. Quick on the road too for a tire that big and don’t wear very fast at all. They test well on BRR and price is great too.
They do seem to compare pretty favourably to other options like the Race King, Schwalbe RS 45 and the Thundero 44 (which is the tire I have been wishing my bike had room for). I can see some arguing the 45 Terra Speed as better but I’ve heard those wear out rapidly.
Terra speed wear fast, they are fast though
Race Only tire in my opinion
Dylan is 100% in the bigger is fast camp, and for the most part this is the same story that’s played out on the road and in XC biking - initially there is resistance and then slowly everyone moves to the bigger tires and recognizes the benefits. Gravel is doing the same thing. 38s used to be standard and 42 was thought to be huge. Now 42 is pretty average.
By the end of last year we saw Cole Patton on mountain bike tires at Big Sugar. Keegan has been going with bigger tires, and Dylan continues to push hard in this direction using the Race Kings at Mid South.
There are still some outliers though in the pro field. Sophia Villafane won Leadville on 47mm Tracers, which is pretty narrow for a Leadville tire but she attributed the tire choice to part of the reason she won. Others would argue she won because she was that strong and left time on the course with those tires.
The problem now in gravel though is many of the bikes haven’t caught up yet to the trend of bigger tires. A lot of popular models like the Trek Checkpoint are rated to 45mm max (although 47s will still clear), so putting on proper XC sized tires isn’t an option. As bikes get updated it seems this is likely going to change, but for now something like a 47mm Pathfinder Pro is going to be the biggest option for a lot of riders.
Cranks will be the limiter on tire size in the future?
There is a point of diminishing returns. There’s a reason you don’t see 4 inch mountain bike tires in XCO. At some point it’s not faster.
I wonder if in MTB and gravel if the bigger tire is a response to courses going “more extreme.” Gravel with chunk and singletrack is essential old school MTB. So a wider tire makes sense in those conditions. If it is a smooth dirt road the 2+ inch tire may not be the best choice.
Just saw this video from Bike Radar….but their testing “protocols” are pretty suspect.
