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In my current game, I pulled some dimensional hijinks, and now my players have access to a temporally displaced modern car (the driver is a mild mannered accountant from Quebec, who is very confused). They got really really lucky on a roll to see if the car was destroyed in the process, and so have a functioning car.

Long story mostly short, they're looking to see if they can get hold of more gas to fuel the vehicle. I know you can technically use pure ethanol in most gas engines, but for longevity's sake it would probably be best to use actual fuel.

Assuming the car has about 2/3rds of a tank left, is there some magical means by which they could duplicate the fuel they already have access to?

Or, and in my own personal hope, once the reigning power in the land captures this mystical vehicle and seeks to co-opt it for their own use, how might some power with large amounts of logistical resources available be able to reproduce essentially future technology?

As an addendum, while the availability of nice smooth and flat roads on which to drive is a complication, assuming that it isn't in the rainy season there are definitely some hard-packed dirt roads that would lend themselves to cautious driving (the accountant has a very good driving record, cautious driving is more or less a given).

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  • \$\begingroup\$ @mattdm To be completely honest, the situation arose because we had a late entrant to the game who wanted to play an accountant. Something along the lines of Twoflower from Color of Magic. Hence the sudden arrival by mysterious means. I like to let the players exploit mechanics I either intentionally or inadvertently leave in the game world, so I figured out a way to determine if the car would survive the journey. Now I'm looking for some way to let them capitalize on their luck without feeling too oppressed by the lack of compatibility with given rules. \$\endgroup\$ – Baron_Von_Munchhusen Nov 11 at 4:40
  • \$\begingroup\$ @BenjaminOlson Your comment seems like the start of a good answer--maybe migrate it on down to there? \$\endgroup\$ – guildsbounty Nov 11 at 16:44
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5E does not lend itself to this kind of worldbuilding in the way some earlier editions did. NPCs and monsters don't follow the same rules as PCs, and the rules for creating spells focus on creating spells the PCs might learn. From the DMG (p. 283):

Avoid spells that have very limited use, such as one that works only against good dragons. Though such a spell could exist in the world, few characters will bother to learn or prepare it unless they know in advance that doing so will be worthwhile.

A spell that provides fuel for a single vehicle that exists in the world (effectively, a unique artifact) is like this. It might reasonably exist in the world, but the 5E rules actively discourage you from creating it.

That might not be very satisfying, but that's really how it is. If you want to imagine a magical world where this kind of thing is normal and commonplace, awesome — 5E materials encourage that, but they don't really provide anything that supports it.

You're basically on your own, which is both bad news (we can't tell you the "right way") and good news (the way you come up with is right!).

To put this in anecdotal terms: I work in IT and have pretty good knowledge of the technical underpinnings of the Internet. I remember long conversations with another geeky friend about how to duplicate such a thing under the magic item creation rules in 3rd edition (or possibly 3.5 — either way it was a while ago). In 5E, it's a short conversation: the DM decides that such a thing exists.

If you really want to explore the consequences of applying a logical, consistent magic system to arbitrary goals, 5E probably isn't the right game to do so. For that matter, D&D overall isn't really optimized for this. You can make it work, but it's definitely not the design point.

So, back to your problem: it's perfectly fine for you to say that the reigning power puts their top arcanists onto the problem and comes up with a spell or magic item or special ritual. That's basically what the system wants you to do. You don't have to put it in terms of player options.

This answer is entirely assuming that you want to kind of make a big deal in-game about the process of figuring this out, either by the players or other in-game entities. If you want to reduce it to something more simple and hand-waved, there actually is a short bit in the DMG on "Alien Technology". That says:

When adventurers find a piece of technology that isn’t from their world or time period, the players might understand what the object is, but the characters rarely will. To simulate a character’s ignorance about the technology, have the character make a series of Intelligence checks to figure it out.

... but this is geared towards "can you flick a cigarette lighter?", not "can you refine gasoline". For that, Alchemist's Supplies (the tool proficiency) probably give the best existing mechanic to build on. From Xanathar's Guide, in combination with Intelligence (Investigation):

When you inspect an area for clues, proficiency with alchemist’s supplies grants additional insight into any chemicals or other substances that might have been used in the area.

This isn't "inspecting an area for clues", but it seems reasonable that the same could apply. In fact, in my experience, Investigation is one of the under-used skills (if there's a clue, most DMs want the party to find it, so it's usually a formality), and it wouldn't hurt to overload this to also include scientific investigations, especially if you're likely to have other anachronistic tech come up in the game.

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  • \$\begingroup\$ Thanks for the response, perhaps I need to clarify a bit more. I'm less looking for a specific spell of "summon gasoline" and more looking for a creative option not without difficulty that would allow them to extend the use of the vehicle. Assuming that they have access to some of the fuel to begin with, I would imagine there to be some roughly equivalent method by which they could determine its properties. I know that Identify will not allow you to ascertain the properties of non-magical materials, but I would possibly allow a skilled alchemist to assist given time. \$\endgroup\$ – Baron_Von_Munchhusen Nov 11 at 4:41
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    \$\begingroup\$ @Baron_Von_Munchhusen Sure! Totally seems within the reach of alchemy. See Xanathar's Guide: "proficiency with alchemist’s supplies grants additional insight into any chemicals or other substances that might have been used in the area." Totally within reason that you could build on that. I guess what I'm saying is: if this is what sounds fun for your group, you should definitely do it. The rules don't provide detailed support, but it's absolutely in the right spirit. \$\endgroup\$ – mattdm Nov 11 at 4:49
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    \$\begingroup\$ That sounds about right to me. I was mostly thinking of an overall balance thing (obviously given that the car itself breaks a lot of that to begin with), and I enjoy letting the players take some initiative to solve problems not specifically defined as solvable by the system as written. I will definitely let them enlist the help of an existing alchemist/chemist type to get started, there's no way the characters could know enough to begin on their own. \$\endgroup\$ – Baron_Von_Munchhusen Nov 11 at 5:00
  • \$\begingroup\$ @Baron_Von_Munchhusen You could also ask on worldbuilding SE to see if anyone knows whether someone could duplicate fuel in a medieval (oftentimes D&D) setting using alchemy and other available means. Also, they might be able to use lamp oil as you mention in another comment and you could ask the question over on mechanics SE (not sure if appropriate) and it could just be about the effective use of it. \$\endgroup\$ – John Hamilton Nov 11 at 14:24
  • \$\begingroup\$ I remember long conversations with another geeky friend about how to duplicate such a thing under the magic item creation rules I was in a game that was relatively free form for a while (don't think we were using any formal rules system at all) that involved characters getting isekai'd, and I told the guy running things, "If I manage to make a computer using magic, punch me." I spent a really really long time figuring out what magical umbrella I wanted to operate under in order to make that task as difficult as possible. \$\endgroup\$ – Draco18s Nov 11 at 19:28
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Is this a diesel or petrol fueled vehicle?

Diesels will run on vegetable oil pretty much indefinitely and that can definitely be purchased in medieval fantasy settings. Olive Oil, Sunflower Oil, basically any vegetable oil. It might smell like a deep fat fryer but it will run just fine.

Petrol engines are more picky - especially more modern ones - and will be much harder to satisfy. Trying to run on vegetable oil is unlikely to be good for them, although ironically the older the engine the more likely it is to work. There will be fewer complicated things to go wrong and less expectations that it is fed a high quality fuel.

Depending on where in the world the accountant is from diesel engines in a private car may be unusual or common. So determine the make and model of the car, work out the engine type - and then this answer may or may not help you.

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  • \$\begingroup\$ Considering the accountant, almost certainly not diesel. \$\endgroup\$ – Draco18s Nov 11 at 19:29
  • \$\begingroup\$ @Draco18s Why do you say that? Plenty of accountants drive diesels. It used to be the best option if you did a fair amount of long distance travel or needed to haul a trailor/caravan/etc. \$\endgroup\$ – Tim B Nov 11 at 20:44
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    \$\begingroup\$ Only 3% of personal vehicles are diesel (in the US, half that in Europe). So I think I can be pretty confident that the probability of a random accountant having been yoinked out of his timeline and owning a diesel vehicle are considerably smaller than the alternative. \$\endgroup\$ – Draco18s Nov 11 at 21:38
  • \$\begingroup\$ @Draco18s Where are you getting your figures from? In the UK: Diesel cars accounted for 47% of all new car registrations in 2016, which has dropped to 31% in 2018. assets.publishing.service.gov.uk/government/uploads/system/… \$\endgroup\$ – Tim B Nov 12 at 9:37
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    \$\begingroup\$ bts.gov/archive/publications/bts_fact_sheets/oct_2015/entire I misread the Europe figure; "less that 50 percent in Europe" -> "less than half <the US figure> in Europe." \$\endgroup\$ – Draco18s Nov 12 at 14:23
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There isnt anything by RAW that would be sufficient use to your players.

I could not find anything that would readily allow duplication of gasoline, though there was 1 thing I could find that your players might be able to use, an Alchemy Jug.

This item can produce a set quantity of a list of liquids once per day. Amongst this list is 'Oil'. Given how indistinct this is, your players could possibly produce said oil and then refine it into gasoline, but there is a hitch, only 1 quart can be produced per day. At most, you get 3/4ths as much gasoline as you had in oil, so you could get as much as 1.5 pints of gasoline from 1 quart of oil.

The big problem with this is that no reasonably leveled spell would allow you to refine the oil by magic, so you would have to use conventional hardware to do so, even if you could use magic to produce the energy input for said hardware.

As for spells, your players would have two options.

First, they could use Wish, but since creating a mass of liquid is not among the standard options, there is a 1/3rd chance of losing Wish, plus the inherent danger of unforeseen effects, meaning that better choices are present if using this spell.

Second, they could forgo the gasoline, and use Animate Objects, but that requires a 5th level slot, concentration, and only works for a minute at a time.

Upon first glance, it may look like even the spell Fabricate is viable, but there is a hitch

You convert raw materials into products of the same material. For example, you can fabricate a wooden bridge from a clump of trees, a rope from a patch of hemp, and clothes from flax or wool.
You also can't use it to create items that ordinarily require a high degree of craftsmanship, such as jewelry, weapons, glass, or armor, unless you have proficiency with the type of artisan's tools used to craft such objects.

It is debatable whether oil being refined would count as it being the same material (personally I think it is close enough). But, refining is a high craftsmanship process, which means whoever uses the spell would need to have done refining personally prior to using the spell. Plus, the volume of gasoline they could produce would still follow the normal refining ratio, and be limited by the amount of oil they could access. However, 'crude' oil (which is used to produce gasoline among other things) does flow from the ground in some places, and was used in some lamps during medieval times, so with some prospecting (and lucky rolls), someone could find a plentiful supply.

Your players do have 1 chance though, YOU. You could decide that an Alchemy Jug can produce gasoline itself, or allow a reflavored item to produce it (such as the Decanter of Endless Water). Assuming you want them to make much use of the car that is.

As to replicating a 'modern' car, the difference in knowledge and technology is just far too great. But, the basic mechanics of a car could be replicated, notably, by steam power. All that is needed, is a source of energy, and a driving system for the wheels. Given the likelyhood of waterwheels or graineries existing, and of teapots existing, a car can be produced.

Off the top of my head, it would need to be a wagon or carriage, with a ready supply of water (could be Decanter of Endless Water), and a source of ready high heat. As for supplying that heat, I could not find any items or spells to safely and reliably produce a useful flame for a steam car, but if you have a fuel source (such as oil), and someone with Prestidigitation or Druidcraft (or other cantrips that allow ignition of flammable material), then you have everything you need. (and you dont even need to break raw)

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  • \$\begingroup\$ I had actually thought of something similar to that approach, specifically the Alchemy Jug and refining the oil. My concern with that was given the assumed time and setting for 5e (which I am mostly observing) lamp oil would be from refined plant oil or the like. That doesn't quite sound compatible with the RAW for a flask of oil being an incendiary weapon, so I would assume it is something closer in nature to Kerosene by default. Still, your assessment for refining efficiency sounds about right, you'd only be able to get 2/3rds yield at best given optimal resources to hand. \$\endgroup\$ – Baron_Von_Munchhusen Nov 11 at 4:54
  • \$\begingroup\$ @Baron_Von_Munchhusen that is why the fact it is only listed as 'Oil' could work in you and your players favor, since there are numerous kinds of oils, with at least some being good to refine into gasoline or something close enough. Btw, I will be editing my answer for more detail, and to reflect your inquiry about reproducing the vehicle. \$\endgroup\$ – Journer Nov 11 at 14:18
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    \$\begingroup\$ Aside from acid, everything else the Jug produces is food or food-like -- beer, water, mayonnaise, honey. To me, this suggests that the "oil" the jug makes is more likely to be olive oil, fish oil, or some other "food" oil rather than petroleum. Up to the DM, of course, but this would be more consistent interpretation relative to other items from the Jug. Now, if the car happens to have a diesel engine, any alchemist ought to be able (eventually) to produce biodiesel, which requires no alteration of the engine. Whether he destroys the injectors getting there is another question... \$\endgroup\$ – Zeiss Ikon Nov 11 at 14:23
  • \$\begingroup\$ @ZeissIkon Given the presence of the acid, the oil could also be blubber based oil. As for being petroleum, 'crude' oil was used for some lamps in at least some medieval societies, due to its ease to acquire, and its ease to burn. \$\endgroup\$ – Journer Nov 11 at 14:26
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    \$\begingroup\$ As an alternative, if a spell exists to apply torque to an object, you could drive the engine mechanically using that. The vehicle would then never need fuel in the first place. \$\endgroup\$ – Rowan Nov 11 at 15:50
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A level 14 transmuter wizard can create non magical objects of any type.

Major Transformation. You can transmute one nonmagical object – no larger than a 5-foot cube – into another nonmagical object of similar size and mass and of equal or lesser value. You must spend 10 minutes handling the object to transform it.

So they simply need to find such an expert and get their help to create more. This can also be used to duplicate car parts and repair or duplicate the car.

A barrel of gasoline is not notably more useful than oil for most people. Finding a unique use for an item doesn't increase it's price.

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    \$\begingroup\$ What's the value of a barrel of gasoline where no such item exists in the world? Priceless? What could you transform that's equal or greater -- a unique artifact, perhaps? \$\endgroup\$ – Zeiss Ikon Nov 11 at 15:23
  • \$\begingroup\$ An object useful only to the holder has only sentimental value, nobody else would pay much for it! Most things are therefore objectively more valuable than a barrel of gasoline.. \$\endgroup\$ – Rowan Nov 11 at 15:49
  • \$\begingroup\$ Probably not much more unless people want gasoline en masse. \$\endgroup\$ – Nepene Nep Nov 11 at 16:30
  • \$\begingroup\$ @zeissikon that's the beauty of this answer. The gasoline gallon will cost as much as the DM (and the plot) demands. \$\endgroup\$ – Mindwin Nov 11 at 16:40
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    \$\begingroup\$ I would also strongly, strongly recommend thinking hard before you let player treat "container full of non-solid I specify" as an object for the purposes of spells. That's a short road to players turning things into "equally valueless" materials that are amazingly lethal. Like turning a pile of junk into a cannister of chlorine gas. \$\endgroup\$ – guildsbounty Nov 11 at 16:50
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Have someone invent wood gas

Wood gas is a fuel generated from wood, charcoal, and other similar organic materials that can be used in place of gasoline in an internal combustion engine. According to Wikipedia, it has historically been used (and is still used) in situations where gasoline was scarce, such as during WWII and in remote areas that don't have access to regular supplies of gasoline, so there is certainly precedent for using it in similar situations to the one your players are in. A wood gasifier can even be built small enough to attach to the car itself, allowing it to generate its own fuel from a steady supply of wood chips, sawdust, charcoal, etc.

This is probably the best option for the players, since building a gasifier onto the car makes it independent from any sort of fuel infrastructure. They will probably need the help of an alchemist or transmutation wizard in order to develop the process and build the gasifier. Note that you might have to do some research about what kind of engines run best on wood gas and retcon the make and model of the car appropriately.

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    \$\begingroup\$ I can see this working if the accountant from Quebec knows how to build a generator -- or even knows that they exist! -- but Forgotten Realms, for instance, still consider Air a single element. Generating a "gas" that can be burned in an engine is centuries of concept ahead of their level. \$\endgroup\$ – Zeiss Ikon Nov 11 at 16:48
  • \$\begingroup\$ @ZeissIkon Yes, it definitely depends on the world setting whether this is a realistic option. But also keep in mind that a transmutation wizard NPC might be able to produce wood gas from organic material through magical means, perhaps a Fabricate spell. \$\endgroup\$ – Ryan C. Thompson Nov 11 at 16:53
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    \$\begingroup\$ @ZeissIkon Also, in terms of the 4-element system, you could justify combustible gasoline with some magitechnobabble like a "stable alchemical compound of air and fire elements". \$\endgroup\$ – Ryan C. Thompson Nov 11 at 17:00
  • \$\begingroup\$ @RyanThompson Or possibly water and fire, considering that gasoline is a liquid (albeit one with a very high vapor pressure). \$\endgroup\$ – Draco18s Nov 11 at 19:34

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