Not sure if you will notice a change in MPG

Your PCM has a high octane spark table and a low octane spark table. When using regular gas it will use the low octane spark table to prevent knock. Many people have used regular gas on stock s/c V6's without issue. Since the low octane spark table has greatly reduced timing versus the High octane table you notice an increase in performance switching to premium.

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04 Indy SS build #972
XP, TOG's, Lightning injectors, 2.8 pulley, 105 CC springs, Wizaired, 3.29 gears, tripleedgeperformance ported blower, machined LIM, 180* tstat, RS12YC@.053, FP rewire, Alt rewire, stage 1 V boost, PRJ wires, Stage 2 IC