Damn, I'm good.
So, Fiona is kinda like the Dreamworld version of the Darkness.
One has to wonder, however, if she does actually have to know how a car works to make a working car there. How a car works here in real life, anyway. She could conceivably dream up a car that works on fun and bubbles. Since it's dream logic running everything there and not real world physics, there doesn't seem any reason why that wouldn't work. She can obviously whip up a pepper storm, so maybe she should make a pepper powered car.
But the pepper brings up another interesting bit which somewhat underlines my previous paragraph . . . Fiona created pepper, but does she actually understand how pepper fundamentally works? It's possible that she's aces at chemistry and botany and all that, but it's also just as likely that because she understands the basic properties of pepper, she's able to whip it up without needing an advanced degree in pepperology. Or, at least, whip up a substance so much like it as to make little to no odds. Consider that the pepper storm was making everyone sneeze, just as common lore claims it does . . . in reality, there is little to no proof that pepper is in fact any more effective at causing sneezes than any other dust-like substance, and perhaps even less so since most pepper isn't ground as fine as most sneeze-producing dust clouds. But since it's so ingrained in the general subconscious that pepper makes people sneeze, it is one of the properties that Fiona made prominent in her own dream-pepper. This would seem to support the idea that Fiona could just as well create a car that operates by shouting very loudly at other people on the road, as so many road-ragers seem to believe.