why 2001 Honda odyssey use 12 miles per a gallon?
 

i have change the whirs and plugs and air filter

has the car been chiped or had the ECU re-mapped on the cheap by any chance, if so the chips most people fit will slowly kill your engine, and a badly re-mapped ECU will do allsortd of unwanted stuff, it will reduse fuel ecconomy, lower or raise bhp but dramaticly reduse engin life, if it is this, you mite aswel get it re-mapped proffecionaly instead of puting back to standard as it wil increase bhp, engin life and fuel ecconomy, i dont no why they dont do it as standard