We've spent some time on this blog in the past measuring the risk/reward level of the OD&D dungeon wandering monster tables (conclusion: as written, in total they're murderously lethal; even Gygax in AD&D massively ramped down the danger level). It recently occurred to me to ask a similar question about the OD&D wilderness encounter tables.
A somewhat theoretical difference is that while the dungeon tables have "levels" which theoretically relate to level of power of the monsters there, and suggested level of PCs adventuring there, the wilderness table don't come with that same packaging. Instead (obviously) it comes distinguished by "terrain types". We might assume that plains are designed to be safer than woods, and woods less dangerous than mountains, etc., but are they really?
What I did was go through all the entries in those tables and compute average Equivalent Hit Dice (EHD) from each type of encounter, using EHDs estimated algorithmically as shown in the
OED Monster Database (code on
GitHub) For example: Working bottomup, here's the subtable for "Typical Men" (i.e., the default for men in most terrain types):

OD&D Wilderness Subtable: Typical Men 
The "grand average" of all the encounter averages in the rightmost column is 122, but that masks the bimodal structure of the table. These encounters split neatly into two halves: six are with mass groups of men in the range of hundreds (with a host of leadertypes, including a Superhero or Lord for any group 100+; I estimated the EHDs for this by adding 25%), while the other six are with small parties of 212 men and a single NPC (like a Superhero or Lord). The average total EHD for the first category is in the 200's, while for the second category it's in the 20's. Clearly there's a big difference between meeting one Lord and his 200 soldiers, versus another Lord and his 10 soldiers.
This was done for all the different subtables, and "grand averages" compute for each resulting in the following:

OD&D Wilderness Subtables: Average EHD 
Somewhat similarly, there's a big bifurcation in the danger levels of some of these subtables. For the Men and Giants (i.e., humanoids) tables, average EHDs are the range of 100+  specifically those tables which can produce bands of men, goblins, etc., grouped in the hundreds (30300 men/orcs, or 40400 for kobolds/goblins, etc.). For the other tables, average EHDs are only in the range of 2050 or so.
Finally, we can turn to the toplevel table, which serves as a function from terrain type to the different subtables, and see on average how dangerous each terrain type is on a perencounter basis. We get this:

OD&D Wilderness Encounters: Terrain Table 
What I've done there is compute the average encounter danger across all results for a given terrain type, and then divide by 8 (an assumed large PC party size?) to come up with a rough "suggested PC level" for adventuring in that terrain. Some of those assumptions can be easily debated, but at least it gives us a normalized basis by which to compare different terrain types.
The result is that on average, there isn't that much difference between the various terrain types. Rounded to the nearest integer, the Clear type suggests maybe 9thlevel PCs, while Woods, River, Swamp, and Mountains are only one pip up from that, at 10th level. The City is 11th (because technically it generates more bands of 100s of bandits and brigands from the "Typical Men" table, however unreasonable that may seem), and the Desert table is 12th level, somewhat more dangerous (again because the "Desert Men" table skews more towards 100s of nomads and dervishes).
So this series of averages is a somewhat rougher analysis than I've done for dungeons (which have been given a complete simulation in software at the level of individual fighters adventuring and gaining experience in separate encounters). The overall distribution of encounters is not entirely clear, although it's trivial to guess that the tables with fewer Men and Giants encounters (River and Swamp) will have less variation than the other tables. Here are some other factors abstracted out by this rough analysis:
 No modifiers are made for parties with special equipment (horses, ships, underwater, etc.)
 No distinction is made for parties that may be parleyed with and turn out to be friendly (likely dependent on alignment).
 Dragons and lycanthropes do not have family/pack structure simulated (which mandates presence of some immature figures, but also makes adults fight more fiercely).
Another thing that the numbers above overlook is that while the average encounter is roughly equivalent across different terrain types, the
rate of those encounters is not. E.g.: Compared to Clear terrain, in Woods the party takes twice as long to cover a distance
and has double chance for encounter each day; and in Mountains both time and encounter chances are tripled, etc. That is, for every 1 expected encounter for a given distance in the Clear, in Woods you'll expect 4 encounters, and in Mountains 9 encounters, for the same distance traveled. Ultimately that's where the real difference in danger levels comes from in this system. (On the other hand, with only one encounter per day, casters can unload their entire firepower capacity on each one, giving some buffer against that added danger.)
Finally, this project suggests a significant limitation to the overall attempt at using our EHD values in sum to balance against total PC levels. Here we've come up with a rough suggestion that OD&D wilderness encounters are, on average, a fair fight for a party of eight 9th or 10thlevel PCs. However, we can look back to our experiences in Outdoor Spoliation games using this system, which we've run with fairly large parties of around the 8th level; at least four times we've documented battles with groups of men and goblins in the size of 200+, and not had a single PC fatality in those encounters. (By the numbers a group of 200 bandits should be ~250 EHD, so for a eightman party we would have suggested they be 250/8 ~ 30th level? That's clearly not right.) This points to a likely breakdown in simply summing EHDs, especially for very large groups of lowlevel monsters, versus PCs with highlevel magic (not currently simulated in our program), very low armor classes for fighters, etc. It may be interesting to reflect on the exact magic used by players in those mass battles in Outdoor Spoliation sessions
One,
Two, and
Three.
Full spreadsheet available here for the tables and calculations shown above.
Edit: Consider Arneson's rule in
First Fantasy Campaign that (as I read it) wilderness encounter numbers are really for full lairs only, and encounters outside will only be 1060% of those numbers (average 35%). If we take the charts above and multiply everything by 0.35 for expected outsiders, then the equated PC level (parties of 8) becomes 3 or 4 in each terrain. Which is kind of interesting, because reportedly at the start of Arneson's games everyone got Heroes from Chainmail  fight as 4 men, D&D 4th level  or else Wizards (I presume lowlevel, likely 4thlevel equivalent?).