I remember sitting at the dinner table when I was but a wee lad. My Father who worked at Sears in the Automotive Department. selling shocks, batteries, tires....
Anyhow as we had our dinner he would expound on the day's events at work.
This particular night he told us of a customer who had a Pontiac Catalina with a big block 400 CID engine. The Customer had came in for the annual tune up. He was boasting that his car was getting great gas mileage. 30 miles to the gallon, great.... This was absolutely unheard of in the early 1970's. The Mechanic, hearing this boast, looked at the serial number on the Carburetor. Called a parts store for a new one, took the old one off and put the new one on.
Why? As I understand, GM had a certain number of experimental carburetors that were mistakenly put into the production line. An oil company, Union 76, had a $500 Bounty on them.
Now some will say this is all urban myth. Believe what you will. My father wouldn't lie. I feel that he was some what discussed at these turn of events.
My point is, the ability or technology to make a large engine provide mileage as a small engine today. by rights, we should have cars and trucks with the mileage above 150 MPG. Sadly someone did write that Gasoline is a cash cow in regards to taxes.