Man, you could have one problem or many small problems.

Your idle could be bad because you have a bad engine wiring harness, bad spark plug wire or bad connection at the spark plug boot, spark plug gap too wide, etc.

Your mileage could be bad due to a bad O2 sensor, but then you ought to have a tripped check engine light.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".