In the mountain bike world it is common to use different tires on the front and rear. The choice is usually to put a grippier/more aggressive tread on the front because it is better to have the rear slide first than to have the front end wash out.
So why not do the same on gravel bikes? There's no cost savings to buying tires in pairs. The front and rear tend to wear at different rates, which is another reason to choose and replace them individually.
I suspect it's because in the road bike world, cornering traction usually isn't an issue -- almost any tire is adequate for either end for the way most folks ride. We're not used to thinking about optimizing the front/rear handling balance.
A lot of people do that in the gravel world. Most people don't talk about it but if you go back and search some tire threads you will see that. The discussion I remember was now WTB Raddler on the front and another WTB smooth tread on the back. Another thought is that the racer like the smooth tread on the back for a faster time when racing. Not much a racer here so I like a more bulletproof tire on the back versus one for speed.
In truth most of the major courses aren’t super technical and I’ll sometimes mullet a smaller front for more aero. Aero not a consideration in rear, but greater potion of your weight is back there. So no reason not to go larger in rear for better rolling resistance, narrow up front for whatever minor aero benefit can be gained.
Some of it has to do with the fact that many gravel races have a good bit of pavement. In the mountain bike world, at least XC, you run a knobbier tire up front with more aggressive side knobs. When you are going fast on pavement you generally aggressively lean in turns, if your tires have vastly different side knobs front to rear it’s going to make the bike squirrelly when you lean into those knobs because of the vastly different reaction front to rear.
I like to try different tires, so I rarely have matching sets of tires. My rougher gravel bike with suspension has a Thunder Burt in front with a RaceKing in the back. My all-road more pavement oriented bike has a 46mm Furious Fred up front with a 55mm Conti Speedking in the back. Both combos work well for me.
I was doing this for awhile on my rowdy gravel bike: Racing Ralph front and Thunder Burt rear. But that was only because I wanted to also ride singletrack. In my experience with Mid-Atlantic gravel there's little value in a knobby tire for the front in gravel -- what's it going to bite into? If you're riding singletrack trails or your roads benefit from knobbies -- or it's muddy -- then I could see value in this, but generally if I'm doing a gravel race I want volume and durability and will choose a tread for that front tire that is grippy on pavement (and lower pressure to provide traction on the gravel).
I've been lurking on this forum for a long while. Great stuff here, thanks everyone.
I think having different treads can have benefit. For about three months I've had Panaracer 35 SK TLC (small knobs,with bigger but fewer on sides) on front and 35 TLC (slick) on back. This has worked fine for me. I like having better traction in the front on dirt. I don't really agree with the note above: "When you are going fast on pavement you generally aggressively lean in turns, if your tires have vastly different side knobs front to rear it’s going to make the bike squirrelly when you lean into those knobs because of the vastly different reaction front to rear."
I ride downhill, I'd guess, in the top tenth percentile in terms of speed. I've not found any issues with some knobs on front and slick on the back. I think of it this way, if two knobby tires would handle a turn fine at a certain speed -- i.e., you are not pushing them past the limits of traction, then swapping one of those tires to one with even better traction will not make things worse. If you do push things past traction limits, well then the squirreliness will come from the tire with knobs, not the tire with better traction. But few folks ride bicycles downhill fast enough to push knobby tires past their limits of traction.
I've pushed it hard downhill with my SK TLC and TLC combo over the past four months. No issues. That includes chasing a rider on road bike and two slicks down Flagstaff in Boulder.
Also I've ridden my motorcycles with very different front and rear tread profiles at twice the speed of my bicycle and never encountered squirreliness.
Anyway, if you are interested in running different tires front/rear, then give it a try.
I use a Continental Terra Speed up front, and a Terra Trail in the rear (40mm). When I bought my Gravel Bike, the plan was to swap tires depending upon conditions, but due to tire mounting difficulties, that idea became less practical. Then, I decided to go with just Terra Speeds, but they wear quickly. So, to get even wear, I decided to use the mix combination, which of course provides a bit better rear traction. Overall, a very good combination.
