Rough average figures for fuel burn at AAR heights and speeds with hoses out are 90 kg/min for the A310MRTT and 100 kg/min for the A330MRTT. But those are very prudent figures as on the 2 A310 trips I did last year, the average was nearer 80 kg/min - although the aircraft was nowhere near MTOW on departure and the trips were only 3 to 3.5 hours in length.
Whereas for the VC10 the figure is 125 kg/min!
Can't remember the figure, but a Gnat cruising at high level used tiny amounts of fuel per mile as it had a turbojet, not a turbofan, and an efficient swept wing. Contrast that, if you will, with the Jet Provost T 5 when we started thrashing them along at 300 knots and 250 ft. It had a thirst like Oliver Read - the only thing moving faster than the fuel gauges was the 'g' meter in even the slightest turbulence!