What year is your 300E? How many miles?
Did the bad mileage start before the tune up or right after?
Have you replaced your O2 sensor .... how old is it?
What plugs did you put into it? Did you check the gap on them?
Often using Techron in a less than full tank of fuel can clean injectors quite well.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)