The best mileage I was able to attain was 26.2 MPG. On average, I got about 25, same as you. One interesting thing to note...compared to other 300E 5-speeds I've driven, yours feels stronger
The 5-speed 300E's were essentially the same as the auto models in terms of fuel injection. The only real difference is the (.80) 5th gear which lets the engine turn a leisurely ~2900 RPM at 80 MPH.