Maybe the more expensive fuel leaves less money in your pocket making you lighter, thus the increased FE from weight reduction?
If you pay with a card, then you have less on your account, so you buy less when shopping, reducing cargo weight, etc.
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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