OK... I have a base 04 Vibe (auto). I don't race the engine over 3,000 RPM. My first fill got me 44mpg. 2nd fill was 28 mpg. Then it dove down to 20mpg at approx. 1,000 miles. It is slowly creeping back up I go 2,000 miles now and am only at 21.6mpg. Mostly city driving.Is this normal?