Let's say my PC get a chance to access some modern vehicles (cars and tanks) as their findings from a ruin, and they try to drive them. What skill should I ask them to check? Maybe first try Knowledge (Engineering) to figure out how it works (maybe by the wizards) and then try Ride to actually drive it (maybe by the fighters)?
PS. Those PCs are not from modern world so they don't have any knowledge of how "this piece of metal" should work.
There are some official rules for operating vehicles. Arms & Equipment Guide has some for ships (both wind-powered and oar-powered) and wagons/chariots/other things pulled by animals. Stormwrack also has rules about sailing, similar to the Arms & Equipment Guide rules but slightly different.
Anyway, these sources provide a bunch of maneuvers you can perform (mostly relating to dealing with difficult conditions or terrain; “you don’t have to make a skill check just to drive along the road”), along with skill check DCs for doing them, and what happens if you fail. Unfortunately, the skills that are recommended for this purpose are Handle Animal—for wagons, carts, chariots, and the like—and Profession (sailor)—for ships with oars and/or sails.
And even though there’s self-propelled vehicle magical augmentation for land vehicles in Arms & Equipment Guide, replacing the draft animal otherwise required to move it, there isn’t a suggestion for what skill that should require. As written, those vehicles would still require Handle Animal checks.
Anyway, while Handle Animal kind of makes some sense for animal-drawn vehicles—as would Ride, and in fact I personally allow Ride for that—there isn’t really a good answer for vehicles that don’t require interacting with a living creature. The suggestion of Profession (sailor) is particularly atrocious, in my opinion. In a world where someone could actually learn and practice these skills, Ride would probably be the closest thing, but I’m not sure the skills one learns to ride a horse are going to transfer to a car or tank, so Ride, at least by itself, seems wrong here.
Problem is, nothing else really seems to fit very well out of the box:
Knowledge (architecture & engineering) might help you figure out how these devices provide motion, but it’s not really relevant to the actual practice of driving one, beyond the absolute basics (what the steering wheel, pedals, and gear shift each do).
If these ruins have a manual, Decipher Script might get more practical information. I strongly suspect that a vehicle would become useless and irreparable before the manual would decay to the point of being unreadable, as long as the manual wasn’t left exposed to the elements, so if the vehicle still works the manual is probably fine too. This probably requires magic either way, though, unless something “ruined” these ruins rather recently.
Profession makes no sense when these are just-discovered vehicles that are not related to any extant profession, and anyway Profession as a skill was a wholesale mistake and trying to band-aid it by attaching vehicle rules to it is awful. Why should this be Wisdom-based of all things?
Anyway, I have not personally worked with vehicles per se, but I have had things come up in campaigns where existing skills don’t quite seem to apply. The expected thing, within the system, would be a feat—but I don’t like that answer, because feats are dear, rare things, and anyway a character may well not be able to choose a new feat for several levels, which is awkward. I have used bonus feats for this purpose—if the PCs make a point of practicing with the vehicles a fair amount, I might award them a bonus feat that lets them use their Ride ranks. But this almost has the flip-side problem, where it becomes “free” aside from some time, which may or may not matter in your campaign (and how much it matters is wholly unrelated to the actual value of being able to drive these vehicles).
So my solution is to sort of crib off of Complete Scoundrel’s skill tricks, which are kind of like mini-feats that you can buy for 2 skill points, rather than requiring a dedicated feat slot. You can buy a skill trick at any level-up, just by spending the 2 skill points (but only one skill trick per level). They’re also usable only once per encounter, which really doesn’t make much sense here. So this isn’t a skill trick, per se, or at least not exactly a skill trick in the Complete Scoundrel sense. But spending 2 extra skill points to allow you to use Ride with one of these vehicles—which you can only do at the level-up after you’ve begun to interact with the vehicles—seems pretty valid.
The other option, which I have used for something that does seem pretty strong, is to create a new skill—Drive, say, instead of Ride. I would usually make that always-a-class-skill-regardless-of-class for those who have discovered/practiced with the vehicles (or whatever). Now instead of requiring 2 skill points, it requires as many skill points as you want to make it work. I don’t love this answer, though, particularly for vehicles: you can’t use a vehicle and a regular mount at the same time, so any ranks you already had in Ride are now useless, and also, anything that interacts with Ride now doesn’t interact with Drive, or maybe it does but you have to decide how that works. You can even go further by creating separate, Drive-specific versions of Ride-based feats and the like, but this just creates more of the same problem, where characters who had already invested in Ride have now wasted those skills, and have to restart from square 1 with vehicles. Arguably realistic, but not fun.
So I suggest that you use Ride, but you require some appropriate investment—time, maybe some extra skill points, maybe a feat but I don’t recommend that—before Ride works with it. Without the investment, you’re left making raw Dexterity checks to try to react fast enough and correct for what’s happening when you pull a lever or push a pedal to get the thing moving in the right direction.
My guess is that either you use plain attribute checks, int + dex f.e. if somebody just tries figuring out how the vehicle works. Ofc. it would be a very hard check, and maybe they have to try multiple times.
Or, let them find some manuals and then they need to find a way to make something out of this. You may end up studying the internal mechanisms of an otto-engine together with part tables and repairing instructions. This is of course completely wrong to study if you just wanna know how to pilot this. At a long shot, I'd give the players time during low-agenda sections, f.e. if they wait out the winter in a small town because it's just too dangerous to cross a mountain section during snowfall. Let them study this and give them the opportunity to learn a skill, possibly at level up, just like they would learn a new language. It's a lore/language/technical skill. I can see knowledge skills from pathfinder 2 before my eye, but i'm not sure if anything like that exists in dnd3
That beeing said, since you are in a fantasy world, the motorcycle might just happen to have an infernal core with a death rider theme. Think of 80's trash motorcycle tv-shows. The vehicle might need attunement or something like that, but otherwise could just function like a companion, maybe with it's own personality
