I think the short gearbox from nubira is causing the high consumption if you usually drive over 100km/h.
I have the same on my e36 (100km/h at 3000 rpm) and consumption is between 9 and 10 l/100km if i go 120km/h and above.
I also had the f16 CR (short one, same as nubira) on espero and i had the same consumption as the bmw (between 9 and 10 liters/100 km), with the longer one that i have right now i have around 8/100 km almost 2 liters difference on a 2 liter engine .
Keep it under 100km/h and it will be ok or replace the gearbox with the longer one from cielo/espero/leganza 🙂 .