While flying along on inner tanks the totals in the centre gauges were showing a decrease and the inners still full. Attacked target and turned back only having 170 galls left in centres and did not know if we could use fuel showing in inners. When only 20 galls left in each centre tank informed crew to stand by to abandon. Carried on even though tanks read zero. 2nd Pilot switched gauges of and on and inners flicked back to 450 galls.