too much engine braking and excessive fuel consumption

[precaution]

i have an old(fourth generation before 2005) and low performance bmw 316 at its 40 000 km (its been in garage for some time after it's done 19 000, then i bought it, so everything rubbery is problematic if that will help), i feel like it burns more fuel then it should considering its performance , since its almost 20 sec. from .naught to 60 , it has a more noticeable engine braking compared to my 2005 passat 1.6 (almost twice as effective), and a higher fuel consumption, when i stop the engine when idling, the rpm needle (or whatever it is, not a native speaker) drops to 0 immediately like in a millisecond or so.. no inertia no nothing, oil and everything is regularly checked up but i think there is something deeper going on. do you have any ideas what could the problem be?