318 or 360

in a mates 70 coronet we swapped a stock 318 (bar headers and v/s 600 4 barrel) for a 'built' 360 (with same headers and a 750 v/s) and mileage went from 25 ish to 15 ish (10 ish towing a caravan), no other changes. performance was vastly improved mind.......