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According to the EPA, and seemingly everyone on this board who has one or the other, they both are so close (within 1mpg) that the mpg would not be a deciding factor in buying either one... whatever you get out of it is really going to depend on your driving.. i've managed to get over 26 hwy on mine but the way i was driving it at the time was using cruise on completely flat land the entire trip for almost 600 miles w/ no hard acceleration.. i've never see it near that high since then.
Owned a 00'GS4 for 21/2 yrs and an 01' GS3 for 2 months. Gas mileage is almost the same, the GS3 gave maybe 20 miles more per tank , the extra 80hp/tq in the GS4 is worth the extra gallon of gas. The GS4 motor/tranny is much much smoother.