Mpg 318

Have a 1971 swinger 318 5.2l. Engine rebuilt. New plugs plug wires electronic ignition PVC valve replaced. Rebuilt carb which is a 2 barrel holly original carb. Seems to run ok but I do expect alittle more performance. Only getting 10 mpg on highway I think 15 to 18 would be right with today's gas. Any ideas as to why such poor gas mileage. Vacuum at ports are good also