Any modern engines or engine builds for modern results?

One thing about gas mileage it's hard to recoup what you spent to get it most of the time.

Look at my jeep gets 12.5 mpg average.
I spend $50 a week in gas so $2600 a year.
Now if I got a car the got double I'd save $1300 a year. But mine is an all year round car. If it was a summer only car it would be only $1300 a year anyways and if a sunny day only half that. So if you spend $5000 on making it better on gas it could take 5 to 10 years to recoup.

Even with my jeep being year round daily driver if I went and bought a $5000-6000 car to replace it would take 5 years to recoup.

After a certain point it's hard to gain much mileage. 100 plus years of the automobile they still haven't increase intown mileage that much for similar cars like ours.

And highway yes you can get great mileage with an OD but if you never or hardly see the highway then who cares.

I barely drive highway around here and if I do its only for 10-15 mins.

Cam and gear are the two things that is gonna effect your mileage but also your performance, it's a double edge sword.