The easier and cheaper MPG improvers are
aerodynamic mods
low rolling resistance tires.

aero tweaks to try are
larger front air dams made of easy to replace materials if they hit something
vortex generators like “shark fins”, homemade wheeler vortex generators or commercial 18 wheel truck “air tabs”
rear wheel skirts

after lowering engine rpm at cruise highway speed,
the next bigger MPG gains come from
raising air to fuel ratio from 14.7 up to the range of 16 to 25

non-USA versions of the early years Honda Insight engine would raise air to fuel ratio at highway cruise to 25

It is the USA NOx emissions rule that keeps air to fuel ratio to 14.7
The NOx regs kill best possible fuel economy, and therefore greatly raise emissions of CO2