my 330ci vert always reads 24.7 mpg fro mixed driving 70% motorway and 30% weekend driving takes £70 to fill that gives me 58 litres approx 13 gallons i get approx 300 miles so a proper brim to brim test of full tank gives me approx 300/13=23mpg to be more exact so the mpg computer on the car is only 1-2 mpg out most i got was 380 miles just doing motorway so 29mpg which is more or less what the figures bmw gives dont forget convertibles are heavier so have worse mpg plus yours is a auto so even worse still on comparison i had a 318ci coupe before the 330ci doing the exact weekly journeys £70 would last me 14 days wheras the 330ci last about 12 days if i dont drive hard it can last 14 days too worked out only appprox £5 more per week to run my 330ci over the 318ci
best thing to do is fill your tank to max reset trip computer and see how many miles you get divide by size of tank should be 13 gallon or maybe more that will give you an accurate reading as your trip computer might be telling lies no point putting £20's in when running bmw's atleast a £40 to get u half a tank i spend more filling in small amounts
also the last month i did a comparison on super unleaded vs unleaded on full tanks there was no difference exactly the same mpg 24.7mpg
my mates e46 352i was drinking loads of fuel under 20's untill he sorted his vacuum leak then it went to 22-24mpg its possible you have a hole in the rubber intake hose maybe
thanks for the advice guys, I just realized that one of my tyre is a low profile tyre and the rest 3 are normal, but I wouldn't think that would be a reason for low mpg.
I will send it in for a service and see if it makes a difference. my friend drives 51plate 528i.auto, petrol and he gets similar to what I am getting.