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Tyre choice is all over the place for starters, but finishers generally all have 42 or 45mm with medium knobbles.
Teravail Cannonball seems popular, that's a 42mm tire that comes up bizarrely narrow (38 in BRR test), but gets a terrible review from bicyclerollingresistance, despite Jaimi Wilson winning on them, and Angus Young coming in first before being disqualified last year.
Getaways have identical volume and "should be" better in every way (according to numbers)
There's also a bunch of G One variants and Riddlers are relatively prevalent.
barry-
Challenge Getaways
Those sidewalls "look" quite thin. Presumably that's where they get any measured speed benefits from. Same reason I picked GK SK+ for Badlands - very supple sidewalls 'should' equate to faster rolling but still got the full punc protection for all the rocks.
Teravail seem to do a lot of sponsorship so I'd be wary of those on sponsored rider bikes.
GBDuro has a lot of road so the tighter tread options look like a sensible choice. I'd still opt for something with extra protection because speed benefit or not, no one wants to be pissing about with cuts mid-race
The missus had an older model G-One Ultrabite 2" or something on her front wheel. No issues.
Better riders (better line choice, lighter on the bike) can be faster with shitter tyres so if you're technically a good rider you are likely able to get away with lighter, faster tyres. If you're a fat fuck, that REALLY REALLY hates stopping for flats you'll likely choose more durable.
I've been thinking about GBDuro for a while and for me I'd definitely go bigger than those Getaways but that's for reasons above.
Also, they sound like they're GKs with the super supple sidewalls - that's gonna be a bitch to seat if you do lose the tubeless lock on 'em at any point.
hippy
I always go big as possible but remember mud-clearance.
What have previous racers used? That should give you an idea of where tyre choice is heading for it.