I have a 2007 Infiniti M35 with 97k miles. My average fuel economy according to the car’s computer and Gas Cubby is 17.6 MPG. That’s not great, but whatever. Since January it has begun to decrease with every fill-up (approximately every 8-10 days), to the point that for the last two it was 15.8 and today 15.4 respectively. The graph is steady decrease over those 13 weeks. The one anomaly was a trip to Dallas. Due to Texas highway speeds paired with car’s 5-speed and short gearing though, I only managed 18.8 on the trip.

There’s nothing obviously wrong. The tires are new and stay at 35 psi. I changed the oil with Mobil1 5W20 in March (no effect). Air filter is new-ish (clean, at least). Plugs are OK. Engine comes up to temp so it’s not a stuck thermostat.

It’s almost time for some major service so maybe it’ll work out then. Coolant, trans fluid, brake fluid, plugs, probably O2 sensors. I also don’t have the tools to do a compression test, although it doesn’t seem to be burning oil. I might have that done at a shop.

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But has anyone experience something similar, or have any ideas? It’s ridiculous that I’m filling up the care (with premium no less) every 250-260 miles.