What gives an engine better fuel econimy?

Question

What gives an engine better fuel econimy?

Answer 1

The driver.

I'm not kidding, with everything else the same, different drivers will get dramatically different fuel economy.

Answer 2

a good tune up

Answer 3

good fuel

Answer 4

key off

Answer 5

Smaller cylinders, fewer cylinders, and hauling less load.  You knew that already by comparing motorcycles with cars, right?

There're also things like the shape of the ignition chamber, the quality of the spark...