Most of the speed mods makes the biggest difference at the top end of
the cruise speed, where parasite drag hurts the most. At the much
slower max-range speed, parasite drag isn't as significant because its
proportional to the square of the calibrated airspeed. Because of this
reason, speed mods only has a very modest effect to fuel efficiency at
lower speed.

The best "mod" you can get to make fuel cost more bearable is the
autogas STC, if you're lucky enough to own a model that can get the
STC, and you can get ethanol free autogas.