What makes a Gravel tire “fast?” Tread (or lack of)? Volume (or lack of)?
Ted King had a post on Insta last week where he 42 knobbies and 44 slicks, and this is what he ran for the races he was in, presumably.
https://www.instagram.com/p/CMkcd1-hB5s/?igshid=4k51ovrd9ajg
If I stick within a given brand and I want to pick their fastest tire, should I pick their biggest or smallest within a given line? Or is it more about the tread pattern and choosing the smoothest option available?
I’m switching from Schwalbe 40’s G-One bite (tubes) to Specialize Pathfinder Pro 42’s (tubeless). Haven’t gotten the opportunity to test yet. Will report back. Cheers.
those are 2 of the fastest tires made
I’m curious to see the effect of that flat band around the Pathfinder’s on the tarmac/smoother terrain performance. I’m also inclined to buy another set of wheels with good road tires for summer long climbs.
Wouldn’t the time to change wheels mitigate any gains made on the climbs, though? 
Edit: that said, if they’re REALLY summer-long the gains would be substantial
pick a hill and do a roll down test to see which is faster
pathfinders are heavy
The following website has the best data available that I’ve come across.
It doesn’t have all the tires you’d like to see on there, and some people question how valid the test protocol is for off road tires. But better to have some data vs just relying on marketing claims, etc.
My plan is to do a roll down test on some tires to see which is faster. Issue is my back up wheels are 300g more so its not an apple to apple comparison.
The numbers online are all over the place, but weight seems comparable to Schwalbe’s. The key difference is the rolling resistance. Pooling the estimates, seems like I could be saving 5 watts and the good one knows I need them. 
The fastest tire setup I’ve ever run in a gravel race is schwalbe almotion 40mm tires with latex tubes. Fast, fast, fast. But not for every course! This was a mostly flat course…because those tires are heavy. And it had rained hard two weeks prior…then got windy and dry…then rained about an inch 4 days out…then got hot and dry. So hero gravel…some pavement.
I’ve also got some G-one’s and Gravel Kings (slick and traditional) in my tire stable. I don’t run the G-one’s much anymore because the Gravel King is a superior gravel tire IMO. If conditions indicate I’m gonna need the traction I’ll run the tradition gravel kings…if it’s hero gravel or 50/50 mixed I’ll run the Almotions…if it’s just normal gravel I’ll run the gravel king slicks.
The G-one is sort of on training ride duty these days.
Roll down tests are great for testing tyres on relatively even tarmac, but its a pretty poor test for a tyre designed to be used on rough, loose surfaces.
The whole subject of ‘what is the fastest gravel tyre’ will be completely dependent on the type of riding you do - what sort of gravel and in what sort of conditions. Unless you’re talking about really smooth light hard packed gravel like on a fire road, you could end up losing more time struggling with grip on rougher sections, climbs and corners than you could make up with a super low rolling resistance tyre on flat smooth straight sections. Grip when putting down power, especially on sketchier surfaces, is much more useful to me for gravel riding than outright speed on a good surface (because of the types of gravel I end up riding on).
If you want to do a test, try to find a short local gravel loop that is representative of the riding you do. Do a load of repeats of the circuit, trying to keep your power even. Swap tyres and repeat. It won’t be anywhere near as scientific as a roll down test, but at least the data that it provides will be relevant to the question being asked
So are bigger tires slower? How much so? How about a 35c Gravelking SS+ vs a 38c Gravelking Slick+?
Oh…now we’re splitting hairs! 
I’d go with the Slick. It’s a bigger tire and it’s lighter…plus I’m not a big believer that the SS+ tread pattern provides any advantages.
FWIW I have two sets of Gravel King’s. One set is 38c SK +, the other is 43c “regular” SK. The 38 SK + are rock hard and “feel” quite a bit slower than the 43 regulars. And this is including quite a bit of road time mixed in with gravel riding. I have no data to back it up though.