I reckon a lot of the variation in mpg is between proper brim to brim checks and people quoting the hopelessly inaccurate calculated consumption figure from the computer
According to the computer, mine does 27 to 29mpg around and about and up to 34 mpg on the autoroute. See my earlier post for the accurate figures
Have to say I've never tested the P38 properly so don't know about the computer - got to fill up today, so I'll check it properly this round! However - not all computers are useless.. my BMW used to be within 1mpg and my Alfa used to be consistently accurate to within 0.2mpg! (something which always amazed me - not least because it was an Alfa!!!)
Matt