Your car is an '87, right? Slightly smaller fuel tank than the later cars (10 litres less IIRC). That's good for city, and good for highway if you drive "quick."
For best results I've found that accelerating "briskly" to the speed you want to travel, and maintaining that speed (less than 120 on the highway), I can average 12L/100 75% city. This is an average after about 22,000 km's from my 150km round trip commute.
Can't beleive I've already put 60,000 kms on my car in a year and a half.
S52|ZF|TRM Coilovers 670/895|Staggered 18" 5's