Technically it can, but any time you add more air into an engine, it will need more fuel to maintain a proper a/f ratio. Some dyno tunes can adjust for more part throttle "leaner" conditions to promote fuel economy, but not here for over the counter ECU tunes.
Again, if a car is making more power at a given RPM due to mods, you may need less gas pedal to maintain current speed-hence a minor improvement.
A pulley should add some, due to less parasitic loss-but this may only be visible at higher rpms/engine loads (maybe not cruising at 60 mph, but maybe at 85mph).
YMMV, literally
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