I got a IS300 sportcross I don't really monitor my mpg in city but 16-18mpg based on my at least 60mile carefree weekly mileage seems realistic. Motorway at 74-78 miles I frequently average 32-34 any faster you be doing you go as low as 25mpg.
In the city driving style is key get to the speed limit fairly quick, foot off gas for a moment and cruise that keeps it in a higher gear, coast, anticipate other driver behavior and generally avoid sudden speed changes and stopping. Now with this car you can easily double or even treble fuel consumption the key is to keep the revs low but still maintain efficient burning by driving at higher speed, keep the oil fresh and topped up and if mileage is high consider new spark plugs originals will last the cars life time but I changed mine just to be sure the right stuff is in there, not some cheap halfords replacement (did the same for brake pads).
I got a little girl now and am patiently on my way to get a RX400h and looking to sell or px the IS300 its done 132k and is in very good condition.