'07 M35 fuel economy keeps getting worse every fillup

[chairmankaga]

I've been watching my fuel economy gradually decrease, literally tank after tank. It was never great, but pretty stable around 17.8 MPG. Since late summer last year it's continued to drop to where it is today at 16 MPG.

I've replaced the plugs, the air intake assembly, changed the oil, cleaned the O2 sensor, reset the ECU (using the tapdance method). I'm not getting any fault/error codes, but it seems every bit of maintenance is pointless, or even makes the situation worse.

Is there ANYTHING I might be overlooking? I'm worried something is physically wrong with the engine and am strongly considering just taking it to my mechanic.

The car has 103k miles and has never had any mechanical problems other than the same oil leak/consumption every other VQ motor seems to experience.

[centralcoaster33]

I think with software and consult devices you can watch your engine's fuel efficiency in real time and review logs. I've heard of increases in ethanol to nearly 10% in gasoline, so it could be your supply. You're difference in numbers are so small, without a chart and methodology, one could easily attribute it to the weight of your shoe on the throttle. Do you have a chart where you log your data that you can share?

[Ilya]

Where you located? Winter gas always lowers MPG's compared to summer...but over the course of a year? Do you make sure your tires are properly inflated? Maybe your calipers are causing minuscule amounts of drag on your brakes?