Suprisingly...
The 428 actually got better mileage than the 61-66 390, somehow they re did the 390 for 67 and from then on they got fairly good mileage for a big motor,but my experience with a 428 was good, my Uncle Henrys brother had a 68 LTD 2 door with 428 Police package , it was very good on gas for a big motor, he always said his 1962 390 got terrible mileage..like 10 on a trip, but the 428 got 17 or 18 if you kept your foot out of it, I think it was geared so high it idled at road speed, also had a friend who had a 7 litre convertible, he traded a 59 Galaxie with the top option 300 horse 352, he also said the 7 litre was better on gas...its a odd thing how some cars then did ok and some were awful, that 65 98 olds I had did really well, 18 on the road, but the 70 I had was doing good to get 8 or 9!!