With Wikipedia as my source (oh save me! it's just too fast!). I don't know how to do the conversion to gallons from the RSRBs, so I will do it in pounds instead.
446,030,333 total miles / (123 flights * ((2 *
1,100,000 pounds PBAN) +
1,387,457 pounds LOX + 234,265 pounds LH2))
Where:
PBAN is Polybutadiene Acrylonitrile, or a type of solid rocket fule
LOX is Liquid Oxygen
LH2 is Liquid Hydrogen
These calculations give the Orbiter .9488 miles per pound propellant.
The ISS which the Orbiter services would be significantly better since it is still up there accumulating miles!
The LOX is dense (over 70 lb per cubic foot) and the LH2 is not (around 4 lb per cubic foot), I am not sure where the PBAN falls, but it's bound to be more dense than the LOX.