2006 Toyota Highlander

Engine Performance problem
2006 Toyota Highlander 6 cyl Four Wheel Drive Automatic 10100 miles

Since I have had vehicle I have only gotten about 12mpg, sometimes 15mpg when it is warmer outside. I just had it at dealer who put it on the computer. All tests were good and I had the latest updates. Since I do not use the car for days at a time, could that be affecting the MPG? The tires are the original Toyos (horrible) and I will be getting new ones soon. Any thoughts appreciated.
thank you
March 16, 2008.

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