Richard.
The leaf dropping to grey is normal is an economy thing to do with the computer so it goes like this: When your in stop/start traffic and sometimes around town the computer will sense that your engine is running but you are not moving much and that is not considered "economical" and will turn the leaf grey and give you that 7.5 reading.
When you are highway driving, again the computer will sense that you have your engine on, travelling at a constant speed and have travelled considerable distance which is "economical" and it will stay green and should raise is to 10 (im going off mine which is a 250GT as an example)
so when it has a green leaf, put your foot down and watch the leaf turn grey and you will understand what im on about.
i get 9/10km/L around town in a 250GT
i filled my tank (till no more would go in with BP 98RON, $136 later) and it lasted a week and a half.
also remember that you are in a 3lt V6 hauling over 1.5tonne around aswell and 1/3 of your power is lost on its way to the rear wheels.
hope it makes sense.