My fuel economy went up the last 3 tanks as well on my diesels. It's the change of season, the fuel blend is no longer winter fuel. Reaming the glow plugs will barely impact economy, maybe 0.5mpg if anything.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k