HI there, I’m looking to get gravel tires for my bike for a race hat historically could get very muddy. I’m considering bringing a dry and a mud race day option or simply run the mud ones if not to far on rolling resistance. I have absolutely no idea what makes a tire good for mud or if there is even a category for it. Everyone talks about the fastest tires all the time, but usually in perfect conditions. Please help!
Thanks,
G
Pirelli Cinturato Gravel M would be my choice here - it copes very well with mud, without being really draggy the rest of the time like a more dedicated mud tyre - the centre strip rolls well, and the sides don’t clog too easily, rolling resistance is ok and the handling is fairly agile.
To build on James’ point, what differentiates a mud tire is not only its ability to avoid clogging with mud, but also it’s ability to eject mud if/when it accumulates. Typically, this is done by larger spacing between knobs.
I’d recommend some Rene Herse knobbies. You can dial in the width as they come in a variety of sizes. I’ve had good luck in all conditions with them and they’re pretty fast on pavement.
Good luck 
at your event
Whatever mud tires you get make sure that they are small enough that you have plenty of clearance with your frame. Running smaller tires is probably more important than the tread pattern.
Vittoria Terreno Wet (or whatever they’re calling it nowadays) is the one. Or a CX mud tyre.
Also, you might want to consider size, smaller tires with sufficient grip can work better as they ‘cut through’ the mud better.
FYI, my experience with Pirelli M is that they’re terrible in proper mud - large flat-ish blocks and minimal spacing isn’t a good combination.
I’m amazed by this, and wondering what kind of mud you have.
I found the Cinturato M to be terrible - it tended to clog, and when it started slipping, it went very quickly, with little/no warning.
The M was good otherwise, in dry conditions, but no better than Race Kings or similar.
The Cinturato S on the other hand is fantastic in the wet/mud. It can cope with way more mud than the M version before it starts sliding, and when the S slides its relatively gradual, predictable and controlable.
I’ve used WTB Nano 40mm in a race that turned to slick mud in heavy rain, I could at least keep it upright while plenty of others went over! There was a huge amount of time to be made in just having some traction through those parts of the course.
I don’t have much to compare that with, typically I avoid the wet/mud like the plague and run small knob tires like schwalbe g-one or conti terra speed 
We must have very different mud 
Interesting that we have had such different experiences. I’ve mainly used them in very wet conditions.
I suppose a useful question might be to ask where the gravel race is happening? Local conditions might influence the suggestions a bit.
I agree the S is much better for really thick mud, or something like a Hutchinson Tundra, I just thought it might be good to go for something that offers a good halfway house.