well the thing is that the fuel gauge is connected with the tire rotation in honda city 2009 2010 2011. if u have tank full of patrol and ur driving on cng. it will keep going down and by the time it will come to the half, u park ur car , go to sleep or what ever the next day u start it. it will show u the actual patrol reading. its just that it takes time, to show the actual reading. Its not using patrol while ur on cng, but u have 2007. i dont know about that one, but i think there is something wrong. do not go to local mechanics. go to honda work shop and have it check by them.