How to increase gas mileage when building a car?

Say I wanted to refurbish a 1979 Ford Bronco but didn't want to deal with its bad gas mileage. Is there a way to increase its highway and city mileage?

Lots of luck.

No not really

You can gear it down, like to 3.07 gears. Also remove any weight that isn't needed. But, both of those will hurt if you take it off road a lot. I got my '92 for desert off road, so I just have to accept the 13~15 miles per gallon.

There's no simple or cheap way.

But there are things that can be done.

Have a read of this article.
http://www.theregister.co.uk/2014/10/19/crawling_from_the_wreckage_essay_making_cars_more_fuel_efficient/?page=1

That's with an old Jaguar XJS, which was never an economical car.
But he's refitted it with Fuel Injection, reduced the drag, electric water pump and power steering, reduced the weight of many parts. And now gets 43 mpg.

So it's possible to improve on 35 year old design. But you have to think, alloy, carbon fibre, electronics, 6 speed gearboxes etc. The stuff that makes newer cars more efficient. Then rebuild the old tank with more modern components.