Volvo Repair: 86 Volvo 740 Gle How many KM per tank?, volvo 740 gle, fuel gauge

Question
Just recently got an '86 740 Gle for free.  The problem is the Fuel Gauge doesn't work.  Learned that the other day when getting gas.Thought the tank was empty because gauge was past red line so I began to fill up, too find gas coming out the other end(Tank to full).I reset the Tripometer to 0, so Approx. how many Km should I get per tank, considering the gas mileage has probably decreased since it was new.

Thanks
   Lee

Answer
it should get about 20-22MPG in around town driving.it should get about300 miles on a tank.I prefer to fill mine at about 220- 250 miles.this would be about 10-12 gallons when you wfill it. sorry for the standard system still not on the metric system here.