If this is any help, our '91 300E gets 20-21 MPG with combined 50/50 city/highway driving. When on the highway it is around 70 - 80 MPH. The best MPG was 24.5 at a steady 65 MPH. The worst was 17 MPG in the winter doing all around town driving. The car has 206,000 on it.
A good injector cleaning way to go is to put at least one bottle of Techron Fuel Injector cleaner in less than a full tank of fuel. I have done two before with excellent results. It is recommended that you also change the oil shortly after doing this.
Also I run 32 PSI on the tires, cold. A little firmer ride but much better handling and a little better MPG.
Has it always been this way? How do you drive? Hard, gently? Also consider that your O2 sensor could be on its way out.
Keep us posted,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)