The Question: Is there good reason to suppose that Gal's desire for internal combustion is irrational, and if so, where's her mistake?

Preferences in-and-of-themselves can't ever be irrational. That's just a rule.

If Gal terminally values internal combustion then that's not irrational, just kinda weird.

If Gal instrumentally values being certain that she's using internal combustion because this is instrumental to the terminal value of her feeling more satisfied about the whole thing, then that's not irrational and is pretty normal.

If we couple this with Gal terminally valuing truth, then we can also rule out deceiving Gal into taking a simulacrum for her own sake.