You could "convert" it to 2BBL by disconnecting the secondaries, but 4bbl to 2bbl has nothing to do with MPG (unless it's strictly a mechanical secondary carb, which i doubt) What model holley DO you have?
2 four barrel carbs on an independent runner manifold could theoretically get as good if not better mpg than a single 4 barrel.
Rear axle ratio may be more important to mpg than carbs...
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