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the main reason I have problems with gas mileage is pontiac did a final gearl ratio for the GTs at 3.29 where the GTPs had 2.93. My wifes 03 Impala has 3.05 and gets awesome gas mileage.
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If this is the case then you have answered your own question. Swap transmissions or just the final drive gears instead.
Add me to the chorus saying a swap away from a 3.8 likely will be more trouble than it's worth.