why is my 99 e300 only getting 26mpg?
 

my old man has a 98 e300 that gets around 31-32mpg and I ended up buying a 99 e300 because I wanted to get the same type of economy hes getting. I got a good deal on a 99 black/black e300 for 9k that had 166k miles and chrome AMG wheels. After driving it for a month i'm only averaging between 26mpg or so and i'm confused why i'm getting worse economy on a car thats pretty much exactly the same as my dads.

the only conclusion I can come up with is that the chrome AMG wheels on my car are much much heaver than the stock aluminum ones on my dads car...but could that really make me lose almost 5mpg?

also i have a slight knocking noise when I first start the car up when its cold that goes away when its warm. It sounds like your hitting a hammer on a solid piece of steel at random intervals...any ideas on what that could be?

Driving style
 

I'd bet you've got a heavier foot than your dad!

My '98 gets 25-26 on my daily commute to work which is a combination of 90% interstate and 10% surface roads. On trips it has gotten 31 but it took a lot of "babying".

That car was EPA rated at 25MPG city..............

Tire size & inflation has a lot to do with mileage...BUT the most likely issue is located on your right leg! Try switching cars for a tank & see if the car is the difference.

My mileage usually works out within the 23-27mpg range.

Only time I saw over 30mpg was driving on the highway at a steady 100km/hr (60mph) for 4 hours. I'd rather walk!

amg wheels are not only heavier, but I'll bet they are wider, and have a softer compound tire on there, I bet you have low tire pressure, you drive harder, and he fudges the figures when he computes mileage...

That seems about right for city driving on our model. I get a consistant 33-34 on straight highway at below 70mph.... sometimes a little less at higher elevations. Highway driving is about all I do and the car seems to benefit by it.

When you get a chance you may want to pull the intake manifold and clean out all the carbon gunk that has accumilated in the manifold/egr and intake ports. Your car may be a little "air starved" and running rich which leads to more carbon gunk build up....its a vicious cycle! Your diesel is 10 years old and this maintenance should have (I say should have) been done at least a couple times already.

I second on the Tire and Wheel combo. Heavy, wide and low profile soft compounds do have a big effect on mileage.

But a intake cleaning is a good idea.

i guess the guy I got it from had the intake already cleaned out and had new plastic fuel lines installed, along with new motor mounts.

the thing is I drove my dads car for 2 months before I bought mine when he was overseas on vacation. With his car I was getting a consistent 31-32 mpg doing the exact same driving i'm doing now with my car. He also has a much heavier foot than I do (its funny how everyone assumes i got the heaver foot than my old man hah)

Good! if that intake mainifold/egr cleaning has been done your one step ahead of the game. (its not difficult just messy!)

As others have said....take a look at the type of tire and pressure that your dad's car is running on and compare it to your tire/wheel combo and pressure.
If you have a wide/ high perfomance compound tire running at low pressure on yours..... it is probably robbing 1 or 2 mpg.

Take a look at the recommeded tire pressure for your vehicle (should be on the inside of the fuel door). Normal load pressure from MB is 27 (cold) front and back (which is way too low to me) and 28 front 34 back with maximum load. As I have said before....car manufacturers do not make tires! Pick a pressure between the certified maximum pressure on the tire sidewall and the vehicle's suggested pressure which should come to around 35 and adjust up or down a pound or two as needed. My touring tires are certified at 44PSI. I run my E300 at 38PSI all day long on the highway with 300lbs of samples and catalogs in my trunk and back seat with no unusual tire wear or handling problems. Even my snow tires I run at 35lbs
You may feel the road in the seat of your pants a little but you will get better economy.

Hope that helps?

Have you compared tire circumfrence between yours and your dads?

I drive 110 miles per day; 80% freeway and 20% city/slow-n-go. I get 30-31mpg consistiently with stock tires/rims and 32psi.

mine are actually a little smaller which would mean its showing that I get artificially worse mileage than I actually show because at highway speeds it is showing that i'm going faster than I actually am by a few mph

Also means that if you and your dad both travel the same speed, that your car will be less economical because its pulling higher rpm's than his.

What size are the tires you are running on the rears?

When I first got my '98 I was getting much worse mileage than I now get. It took about a year of "spirited" driving to clean out all of the crud from the intake and exhaust and get it to the point where it now routinely gets around 30 MPG. If yours was owned by someone who drove it like a little old lady that could explain it too (though with chrome AMG wheels it seems hard to believe the PO was an old lady).

Over-size AMG tires/wheels will certainly have their penalty. FWIW, I recorded three successive tanks this summer @ 29.5 (US) mpg which is the best I've ever done. If I drive the car harder, 26.5 or so becomes the norm.

These cars are not fuel economy champs by any means. My father has a 100% stock 1998 diesel and his fuel economy is very similar to my own. It's not spectacular.