I agree.
This is why I track all my cars mileage just to get a hint something is wrong.
The way I do it, which makes it easiest for me, is to just run 10 gallons per fill-up.
Calculating MPG is easy!
My '87 Turbo does a consistent 12.5mpg now.
This is basically all city driving, as work is only 4 miles away for me.
When I jump on freeway, it'll hit 20mpg pretty easily.
Ported 13BT, hybrid turbo, Haltech E8, Crane Cams HI-6 on leadings
My best record has been a Sacramento, CA to Las Vegas, NV run...
400 miles between fill-up's
~13 gallons per fill-up, due to smaller S4 gas tank (16.6 gallon max)
That nails exactly 30mpg.
All freeway / highway, keep tach under 3,800RPM, average speed around 80mph
Car back then was running reprogrammed ECU, small hybrid turbo, Crane Cams HI-6 on leadings, same ported 13BT motor.
-Ted