I'm getting under 20 MPG with City/Hwy combo driving and want to know why i'm not getting mid 20's at LEAST. I've run Red Line Diesel Fuel Catalyst, and changed the oil and air filter. Any ideas?

 

How many miles on the car? Fuel economy sounds very low. Has the intake manifold ever been cleaned? How's the smoke?

 

72k miles, just a baby! No smoke whatsoever. My mechanic says the MAF sensor going bad would give the computer an error message. (i've read posts about replacing this part)

 

The MAFS failure may be gradual and may not trigger the CEL in all cases.

Does the car peform as you would expect it to?

 

yes, performance is great. do you think i should just change it? the part is so cheap...

 

"Cheap" is relative. I am reluctant to be the one to tell you to start replacing components without being certain there's a fault.

(How cheap are they?)

 

around $160, which in my eyes is cheap for Mercedes based on some of the part's i've had to get! I wonder if there are any other factors which can affect the mileage in a big way?

 

Many things will cause you to get poor "fuel" economy.

You are using over-sized wheels with extremely low-profile tires.
This is costing you fuel economy!
Many owners over on Freds' TDI group have reported reductions of as much as five miles
per gallon after switching from the factory low rolling resistance 15 inch
wheels and tires to the "Frisco" 17 inch factory options from the GTI.
You have done the very same thing!
The tires you are using? The factory installs what is referred to as a low rolling resistance tire.
They do this for a reason.
Do you still have the original wheels and tires? (Hint hint.)

Low tire pressures can reduce fuel economy, but not as much as those wheels and tires.

Driving styles.

Jack-rabbit starts.

Do you take off in regular drive, or do you ease off using the "second" gear start feature?

I have found that my city only fuel economy could be a low as 20 mpg
if I was not careful when taking off from rest.
Learn to start off slowly, and get to the speed you want in each gear, and then let up a little, causing
your transmission to upshift early and catch the next higher gear and take advantage of your
motors' tremendous low-speed torque which is there at the unheard of low rpm of only 1600 rpm!
In other words, try to "lug" your motor, which is actually impossible to do with your car
because of the automatic transmission and the torque converter that does not lock up!

Highway speeds?

At what speeds do you cruise? You do know that excessive speeds will kill your highway fuel economy?

If you hold your speeds down, it is indeed possible to get the EPA estimated highway fuel economy of 36 mpg.

Try changing your driving styles, and see if that doesn't improve your average fuel economy.

Many here have talked about the MAF causing poor driving conditions, and lousy fuel economy.
I could not make mine do that. I tried running with it disconnected. I even changed
my original factory stock one over to a new one that I had that was
an original part for a TDI, and I could not tell any difference.

Motor oils.

Are you running a straight weight oil, or a multiple grade such as 10W-30
full synthetic, such as Mobil 1 Delvac or AMSOIL?
Next time you change, try using Amsoil Series 3000 5W-30 Heavy Duty
Diesel Oil and see if you don't see some differences!

Warming up the car before driving off?

Do you star it up and drive off right away, or let it sit and warm up and then proceed?
Remember, you are getting zero miles per gallon when you are not moving.

These are but a few of the things you might try to improve your average fuel ecoomy.

Good luck.