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Around town, like any v6 I average 18 or less mpg BUT on the highway I get as high as 27mpg average, and that's on the original Dunlop Summer Tires. I would imagine a Grand Touring Tire like the Primacy Tour A/S would get me 30 mpg
If you overinflate by an extra 5psi or so you can squeeze out more MPG and you may not notice much of a ride difference if you are traveling long distances with good roads. I typically average 30-33MPG in my 3GS on my trips back and forth between Houston and Austin back then.