In the 70’s, I owned a 64 Rambler Classic with the 196 inline 6 engine and 3 on the tree. Rambler consistently won the Mobil fuel economy runs. I got about 25 mpg and that was the best of any car I owned from that period.
My father weekly drove from PA to WV & back , about a 5 hour 1 way drive & kept fuel mileage records so the 26 mpg was mainly trip mileage .
Then your dad far exceeded what other people did.
All I know is he had an expense account & had to keep a record of all expenses , motels , meals , traveling expenses , money spent on fuel & miles driven , etc . Once when he was doing his paperwork I asked him how many miles he got to the gallon & he told me 26 . If you’re determined to say he was lying you can go you know where .
My 1965 Dodge Dart with the 273 V8 automatic did on average 17 mpg, and only on long highway drives could break 20 mpg. Every one has fond but distorted memories of past things and events!
I have a fridge magnet that says" The older I get, the better I was"!!!