Is that on regular unleaded or super?? with regular i get about that much, but with super i get about 4-5 miles per gallon more per tank which is another 70 miles or so. Try super and you will see the difference, i guarantee it. Its worth the extra money, and you will actually save in the long run. hope this helps.
I usually see anywhere between 260 and 360 miles per tank depending on how much city/highway driving I do. I haven't filled up more than 11 gallons, meaning I leave about 2 gallons in the tank before filling up again. This is with 134k on the odometer... Car runs great! The previous owner took care of this car.
1993 NX2000 (134k miles - 129k when bought)
1998 200SX SE-R (43k miles)
I usually fill it when it gets low according to the gauge and the orange light starts to flicker which is 240-250 miles mixed driving. It will take between 10 and 11 gallons. This is with the stock ECU, mileage tends to be higher for the JWT ECU b/c it runs leaner.