Isn't there already a gas station inside LSPD? The last time I was a cop, there was one in there. Has it been removed or what?
You can just buy a fuel jerrycan from F7 and put it in the car. Adding such a suggestion will make the server cycle of selling the jerrycans via F7 drop. Adding to that I can see that there're fuel stations all over the map, means that you can just add the fuel jerrycan to the vehicle and move to the nearest fuel station to make it 100%, its not hard to be done. Negative.
This suggestion is by no means new and has been suggested before. M1D0 has already explained everything, no need to add anything.