need help i get 100km with 25 liters of gas

i get 100km with 25liters of gas
can some people here tell me a few thing i like what part 2 change so i can save $$$$$$ some peop;e told me i should get 100km with 14 to 15 liters
i didnt do tune up yet but ill reather do all my repairs at one time so i have less down time it a work van
i get the same mpg as it load or not and i got new wheels
and when i floor it doesnt go it revs but goes really slow but when i give it half on the pedle it moves out good so new trany or rebuild
so tell me every thing u know
and the only mods i have done it lift the back end with larger leaf spring and 2 battery 2 power the stero and ac to dc converter
also any one here has put the front of there van on bags need some info and so i can have it level right now it like the old school car with the back end in the air and the front on the road
can some people here tell me a few thing i like what part 2 change so i can save $$$$$$ some peop;e told me i should get 100km with 14 to 15 liters
i didnt do tune up yet but ill reather do all my repairs at one time so i have less down time it a work van
i get the same mpg as it load or not and i got new wheels
and when i floor it doesnt go it revs but goes really slow but when i give it half on the pedle it moves out good so new trany or rebuild
so tell me every thing u know
and the only mods i have done it lift the back end with larger leaf spring and 2 battery 2 power the stero and ac to dc converter
also any one here has put the front of there van on bags need some info and so i can have it level right now it like the old school car with the back end in the air and the front on the road