Welcome to EM
It's good to hear that you are doing so well with such a car, though an automatic isn't the best choice for long trips at a steady speed.
If your comfort is the most important then not everything will work, but there are still a few things we can suggest. Not going into boattails (which are great for highways!), wheel skirts, etc., then maybe at least a partial grille block for winter?
Oh, and slowing down always works. At highway speeds, even lowering your cruising speed by 3-4 km/h will extend the distance you can do on one tank.
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e·co·mod·ding: the art of turning vehicles into what they should be
What matters is
where you're going, not
how fast.
"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell
[Old] Piwoslaw's Peugeot 307sw modding thread