Gas Mileage issues with 2000 lexus es300?

supposed to give me 19 city/26 hwy. Just started driving it, as it was another family members. I barely got 300 miles on a tank of premium gas (wanted to try premium first)...wtf? when i put premium on my camry, it gave noticeably better gas mileage. Spark plugs are fine, sensors are good, any1 know what is wrong? btw that came out to about 17-18 mpg. I drove 30% downtown atlanta, 20% highway at 5-10 speed limit, and 50% residential driving. I NEVER exceed 2.5 RPM, 30% downtown driving can be harsh but its only 2 miles to school and 2 miles back. Please help :(