What size is the stock I-pipe on the V6 cars? If it's less than the 2.5" then you will definitely see an increase in mpg. It might not be big but even a .5 mpg increase is nice. Winter is not the best time to compare mpg though, every vehicle drops a couple mpg during the winter. I average around 18-19 mpg daily driving in the summer with my LS1 and it drops to around 15.5-16 mpg in the winter. My old 4Runner would average 20 mpg in the summer and 17 in the winter. I picked up a solid 1 mpg with the 4Runner after installing a 2-1/4" cat-back which replaced the stock 1-3/4" exhaust. So bigger pipes will help mpg in the long run. The initial change in driving style may hurt as you enjoy the new sound and get on it more often.
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69 Z28 JL8 4 wheel disc brakes - being restored
09 Silverado Z71
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