1997 318iS auto coupe excess fuel HELP PLEASE

[Tam2010]

Hi , can any one help me with this . I have a 1997 318 iS AUTO COUPE . According to an HPI check all is well including the 48,000 milage . I have had the following work done : oil+ filter change , injector cleaned with the correct fluid . The mechanic says the air filter is good and the spark plugs are good but here,s my problem , I am only getting about 45 miles from 9 litres of fuel ( 2 gallons ) . I have been driving at careful speeds with no harshness whatsoever so I could gauge this usage ( forgive the pun) , Goodnes knows what woud happen if I was to do some faster driving .
If anyone can help with some suggestions I would appreciate it but if you tell me this is about right the car will have to go . it is in lovely condition throughout so it would be a shame to lose it because of this .

Thanks