Does high humidity climates effect/worsen FE?

[MechEngVT]

I think a vehicle's response to humidity depends on the engine and ECU configuration. My truck responds favorably to humidity in cool conditions but unfavorably to hot conditions (whether humid or not). I think this is primarily affected by the spark timing and the knock sensor. Humidity should improve knock resistance and therefore should prevent spark retard. The temperature side of the equation easily overcomes the influence of humidity on my truck, so when it gets hot it doesn't much matter if it's humid I think there are times that the spark gets pulled back. Plus I run my A/C as I really don't tolerate heat very well, and the A/C probably stays engaged more in humid heat.

If your vehicle doesn't have a knock sensor I don't know if you would notice a difference unless in dry conditions your engine knocked, then it may reduce your mileage from preignition opposing crankshaft rotation.