Airflow through the intake ports of the cylinder heads has been improved a great deal. That accounts for about 70% of the peak horsepower gains since 1970.

On supercharged engines
intercoolers have made a big difference.
The scientific discovery that air/fuel charge temperature has a far greater effect on detonation
than charge pressure or
time of burn
led to big hp gains.

Fuel efficiency of engines only has not improved at a fast rate:
0.7% per year,
for a measly 7% per decade.

MPG of vehicles has improved more than the snails pace of engine efficiency
due to low rolling resistance tires and
aerodynamic Cd going from 0.6 to as little as 0.26
and taller gearing approaching a ratio of 40 mph for every 1000 rpm.

14 mpg cars have been around since the 1920s.
The 1937 model year VW bug was a 40 mpg vehicle at 50 mph.
Todays 30 -ish MPG five passenger cars at 75 mph are not really that much better.

Gasoline "real prices" are also more constant than first appears.

32 cent per gallon gasoline
when vending machine Coca-Colas were a dime,
and a Troy ounce of Gold sold for $36
or a Troy ounce of Silver for $3

Is not too far off

240 cent per gallon gasoline
when vending machine Coca-Colas are $1.25
and an ounce of Gold sells for $1280
or a Troy ounce of Silver sells for $17