Mine is 91 model manual. Giving average 10km/L with AC in city.
If you’ve purchased it recently then do the following things.
Tune it up from some good mechanic.
Replace the old plugs with New NGK or Denso.
Change the engine oil. First priority is 10w-40 grade oil. It will give you the good average. Presently I am using Delo Gold Ultra 15w-40 Diesel engine oil and is satisfied with it so far.
Replace the old gear oil.
Fill the radiator with pre-mixed Coolant instead of water.
If its fan is in direct mode, then ask electrician to put it on auto mode.
Ask him to check if PCV value is working OK. If not, then replace it and its rubber pipe too.
Change the hose pipes of the radiator. Upper & lower also. Check the heater pipes if these are also need to be replace, then do it.
Use good Air & oil filters.
Keep the tire pressure 26 to 28psi. But it should be same on all tires.
Ask electrician to check its distributor settings and make sure its correct functioning.
Ask Electrician to make sure its Thermostate valve is placed and working.
Have the service for the brakes, alignment and wheel balancing also.
All the best.