Toyota Corolla Altis Diesel — diesel consumption average is only 15 km per liter

Address:

Valsad, Gujarat

Sir
i have purchased toyota corolla altis diesel in january 2011. it was giving average of 19 km per litre. after first service at vapi nanavati it comes down to 15 km per litre. how ever i have inquired about this to my dealer nanavati vapi he send me to there another branch at surat, i have gone to surat after inspection they suggested me to go for another service before its due time for service. i have once again given it for next service to avail average but again it is giving me 15 k m per litre average.
now nanavati vapi brance says that we can not do any thing for this. kindlly help me and rectify my problem as it is claimed by your company it should give me atleast 20 km per litre insted of 21.43 km per litre average.
i request your good self to look in to matter as your dealer is help less in to it.