Swamped

With 5500 miles I've had my share of fill ups. While Lexus states a 17 gallon capacity, my TX500h has never taken much more than 12 gallons. Even when the remaining range was 6 miles out only took on 12.6 gallons. Once filled, it will indicate 348 Mike range with 23.1 mpg which comes out to 15 gallons. What are others experiencing?

I have a TX500h with 5500 miles clocked after ten weeks of ownership. Driving about 70% highway. Mileage is averaging 23 mpg. What are others seeing?