I need helppppp!!

mi name is yunier from dominican republic, i have a nissan march/micra 997cc automatic and it spend a lot of gasoline, i can ride 37 km per gallons, or the same 37km per 3.7 liters. i conect it to a computer test and everithig is fine, i changed filter,temperture sensor, injectors... all... and i can't find the problem. please somebody help.
How can i know if the car's computer is damage or what's happening with my car.

sorry about the way a speak because i don't speak english very well...

thank you...

you could try a voltmeter on the middle wire to your lambda sensor yunier, it should read about .5v at cruise

frank