This is a bit one sided towards Maxxis.
I have been experimenting with a few combos, and i have found that for-
Dry track, loose gravel: Minion F (designed with ramped knobs for low rolling resistance and channel-cut knobs to increase gripping edges, giving straight-line control and precise cornering) for the front, then Highroller (ramped leading edges roll fast, while the side lugs offer stability at speed and a sharp edge to hold corners. The grooved center knobs provide straight-line control) or Minion R (The Minion DHR features ramped knobs, like the DHF, but the channels in the knobs are designed for braking and accelerating) for the rear. The Minion tires were designed for the often loose and muddy conditions of the NORBA downhill series several years ago.
Hard pack: Minion F for the front, Highroller for the rear.
Wettish gravel: Minion F for the front, Minion R for the rear
MUD: well thats a funny one. depends on weather it`s only mud, or clay shit mud that sticks to everything. If its a nice mud, well Wet scream (They are designed to channel mud out of the tire so that the knobs remain being able to still hook in even when there is mud in the tire) for the front and a Minion R for the rear.
The most common size tire used in DH has been 2.5 inch both ends.
The compounds of the tires are a difficult match though.
Usually a softer compound in any conditions works well, but if the tracks are rocky and you are using a softer compound, like a super tacky, they tend to "disintegrate" thats a bit of exaggeration, but you get the message, so often, using a harder compound increases the lifespan of the tire.
Using a softish compound allows the lugs on the tire to "roll" and hook into the dirt, or whatever it is.
So loose rocky stuff, slow reezay 40A compound, harder pack, can stay 40A but may also be better to go 60A Max Pro, wet mud, definitely a soft compound so it can bite.
Sorry about the length.
Just a thought.
Liam