E61 M57 525d Fuel Economy

i have a december 2004 525d touring - i am a bit disapponted with the economy - at first i wondered if the OBC was faulty but i tried the top upto the topdrive and top up again technique which demonstrated tat the OBC was 5-10 % accurate by the OBC the car does 22mpg round town and 35-37 on a long run - i have never averaged over 40mpg. No faults have showed up on the diagnostics - is this ok or is there a problem ?- i have been informed that it is very difficult to isolate the cause of poor fuel economy. I should of course add thatthese figures  are for 4 people in the car - we probably average out at 60-70 kg each