hey guys, if any of you remember i had a rough idle/jerk problem at stop lights a few weeks back, which was later determined to be a malfunction with the fuel injection system. After getting the car back, I've noticed that the computer has been consistantly estimating my mpg to be around 18 mpg! I do a bit more local than highway drives, and I drive like I have a senior discount card a lot of the times, so shouldn't it still be above the listed 21 mpg? What can I do to determine if I'm really getting only 18 mpg or if the computer is at fault? Thanks in advance.

 

My 530XI is averaging 24MPH, so yes, you should be doing a bit better than that.

 

Yup - for the first year and a half I kept a spreadsheet of my mileage on my 2006 530i manual. Never dipped below 24mpg, saw as high as 28mpg. That's not for certain stints - that's total combined real-world driving - data computed at each fill-up. You should be seeing a lot more.

 

As a check on the computer, have you done the calculation manually each time you fill up? Does that jive w/ what the computer is saying?

 

Answering your question "how do I determine if the computer is wrong":

1. Fill the tank to the brim
2. Reset the trip odometer to zero; reset consumption OBC (if you have prof nav, then just reset the journey computer - this will give you both mileage and consumption)
3. Drive around for at least 250 miles
4. Fill up to the brim and make a note of how many gallons that is.
5. Divide miles on trip odometer by gallons and compare with OBC indicator.

If the OBC is out by more than about 5% and your measured consumption is "normal" (21 mpg or so), then the OBC is at fault (it may be possible to retrim if the error is within +/- 15%). Otherwise, there is something wrong with your fuel supply: poorly trimmed injectors or fuel leak. Or you really have a leaden right foot.

Good luck!