Wheels / Tires
Gravel wheels that will clear 4/2 pot Subaru brakes with 15" gravel tires:
Team Dynamics Pro Rally wheels (opening between the spokes is tight they tend to not evacuate dirt as well as more open wheels)
Evo Corse wheels: http://rally.build/category/products/wheels/
Method MR 501 VT-SPEC (15x7 et48 5x100 for our 04 STIs): you can get discounts from Method if you buy at least 3 sets (consider doing a group buy with someone else)
There are a number of gravel tires that are available but most of them are in 15" diameter. The top level brands like Pirelli, Hoosier, Michelin (hard to find in the US) are extremely expensive. The super soft or soft coumpound will usually offer the best performacne at the expense of longevity, most tire manufacturers will give you a range of operating temperatures and conditions based on the temperature.
Affordable options at a good performance level include MRF ZDM3 (available through https://tirestreets.com/products/mrf-zdm) and Federal G-10 (https://rally.build/collections/federal-rally-tires/products/federal-gravel-rally-tires) with the Federal having softer sidewalls and a bit less performance than the MRF. We switched from Federal to MRF and have been pleased with their performance so far (see MRF compound info in the table below).
Other brands come and go, you might find used Maxxis, DMACKS, Cooper but those brands are not around anymore. You can find some brands like the Yokohama A053 overseas but the shipping costs are pretty high.
For rallycross use, there are lower quality tires that are more affordable like Fedima, Zestino or Indysport. Usually not suited for stage rally usage but they have specific models more adapted to mud or sand that could be seen at rallycrosses.
Snow tires are not really adapted for rally duty but when there is snow and ice, even super soft gravel tires won't give you enough traction. Many brands of snow tires offer competitive performance (check the latest comparison on your preferred YouTube channel to see what is the hot tire of the moment). Tire tractionizing can offer a good performance improvement for traction on ice when studs are not allowed (See video on the right).
The Yokohama A034 was the most common rally snow tire available and it has been discontinued in the mid-2010s. There are still few sets circulating around but they are very hard to find. The Michelin C5 was an equivalent, also discontinued. SUV versions of street snow tires can offer a stiffer and reinforced sidewall compared to the similar passenger car version (see Nokian Hakka CR5 vs Hakka 5) but it is becoming harder to find SUV tires in 15" sizes.
For icy conditions, nothing beats studs if the event allows to use them. The Nokian Hakka 9 or 10 are the best performers at the moment, they are even used as a spec tire for street legal racing by AMEC. If the regulations allow, WRC studs like the Pirelli Sottozero Ice offer the best tractions. Alpha Tires also offer a cheaper alternative to the Pirelli with custom studding.
Gravel tire grooving
When conditions get muddy or sandy, grooving tires to improve traction in these challenging condition might be necessary. We use a Van Alstime G-1000 tire groover to cut our tires. We recommend practicing on used or damaged tires first. You can watch our video below.
Here is a page with information about Pirelli tires: https://www.fourstarmotorsports.com/pirelli-tire-tech
The pictures above show manufacturer recommendations for Hoosier and Pirelli gravel tires. The video below show the recommendations for Michelin gravel tires (video is in French but you can activate English subtitles)