Why is my fuel economy all over the place?


Professional Noob
Apr 26, 2009
1998 Lexus GS400, 4.0L 1UZFE V8. Runs like a top at 206k miles. I routinely make trips from Idaho to Utah and use the time to check MPG and engine performance and such.

-Last weekend I got 26mpg with the cruise set at 80mph. Weather was good, in the 40s and dry. No headwind.

-Same trip today, got 20mpg!! Cruise at 75-80mph, slight headwind, cold and wet and in the 30s.

-On the same trip earlier in the year I got 24mpg, cruise at 80-85mph, slight headwind, hot and dry, in the 90s, with a ton of luggage in the car to boot.

That's a bad enough difference to concern me. I checked over the whole car, can't find any issues.

None of the brakes are dragging, they're all cool to the touch after getting home.

The fuel trims are all within 2% at idle and about 4% rich at 2500-3500rpm.

Tire pressures are all at 32psi.

The only different variable was rain and snow. Could that possibly make a 6mpg difference? Can a headwind even make a 6mpg difference? I would expect if the wind was that nasty I could feel it knocking the car around. I've felt it before, but not today... WTF.
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