The purpose of a throttle
is to create
"Worser Breathing"
to restrict an engine back to a lesser horsepower.

Adding "Better Breathing"
to an engine operating at part throttle
does NOTHING
because the throttle blades will just close a little more.

An engine operating at part throttle
will get better MPG
if you reduce the friction
or if you increase the compression ratio.

Do you want your engine to
"Breathe Better"
at 70 mph cruise?

To do that change the gear ratio
so that the rpms go down
and the throttle blades open up.
The cylinders are now breathing better
(in engineer speak the "volumetric efficiency is higher")
and the friction is less
because the rings are not rubbing the bore walls as fast.
As the piston is going down on the intake stroke
it also is not having to fight against a vacuum pulling opposite.
(in engineer speak the "pumping losses are lower"
creating the same benefit that the MDS on 5.7 V8s does
when 4 cylinders get a more open throttle and 4 cylinders are closed off becoming 'air springs')

This is why nearly every gasoline vehicle now has overdrive top gear.

90% of the bad ideas people have on this forum about MPG
are because they don't understand how throttles work
even though your fathers and grandfathers discovered all about
throttles and left that knowledge to you for free
...if you will just take the time to learn it.