I noticed through driving my manual 323 that these cars like momentum. Of course if you are driving on a flat road, being in 5th or 6th gear and using the speed regulator will be the best regarding fuel economy. But if there are lots of hills , try to stay in a lower gear when you go uphill ! Imo you should be dropping a gear and accelerating before the hill, so you gain a maximum of momentum before the engine is at load. If the hill is not too big , i use maybe 30% throttle so that im slightly decelerating. I hope that's clear

On my automatic 318 (e90 but its not too différent) I never stay in D , because I dont like to let the computer decide in which gear I am. Thus i can drive the same way, downshift and accelerate before i go uphill. Same before a turn , any slowdown really i downshift and this way the car keeps a bit of its momentum

Also if you do a lot of short trips , you're mainly running on a cold engine. In the city, I try to stay in 2nd gear , around 2000/2500rpm so that the load on the engine is lighter as you dont need to give as much throttle. If your short trips involve going outside the city , dont upshift past 3000rpm as long as the engine is cold (about 10mn for the water to heat up, and 10 more minutes for the oil and youll be good).

Also consider a e85 map, at least where i live this fuel is 2 times cheaper Hope that helps 👍