Honestly, I got a predictable 17 on highway trips; I think the speed limit was still 55 so i would drive 65 at most.---A daily mix of street and highway I got 14-15... I kept meticulous records for 5 years of ownership.
This was in a low mileage 1973 450se; Bosch platinum plugs and Mobil 1 oil.
(It is possible the odometer was incorrect)