I'm guessing they are 18" not 18' ;)

What is the exact size of the 18's you are running? Depending on the profile of them they might not be any bigger around than the stock 16s or they might be.

18 feet. I am really cool. It's like a monster truck :)

the larger tires don't account for nearly all of the difference, though they might not help much since they might introduce some error in the speedo/odo readings. Still, to get only 300 miles before seeing the reserve light come on stinks. I'd dump my car in a second if I was seeing miles like that.

Just out of curiosity do you then put something like 17-18 gallons in when the reserve light comes on?

If you're ripping up rear tires, and getting 18mpg, you're not doing that bad.

I get about 30 in ideal hiway traffic. 80mph, no traffic.
I get about 18 in stop and go local traffic.

18 mph and ripping the tread off the rear tires sounds like you're gassing it to me.

I'd get an alignment first, end that question. Then try to lighten up on the throttle to lighten up the fuel bill.

At 142211 miles I put in 19.26 gallons, 321.4 miles later
at 142533 miles I put in 19.65 gallons. 409.1 miles later (I took a road trip)
at 142942 miles I put in 18.89 gallons. 340.5 miles later
at 143283 miles I put in 18.95 gallons.

I have had the car 13 months and this is how it has been the whole time.

I drive very conservative, with very few moments of "stepping on it". Now, my BMW 540, well I might have driven that a bit harder ;)

But the Merc is a cruiser.

Do tell us the full tire size. It can make a bigger difference than you think.

18" wheels make a huge difference on these cars with regards to fuel consumption.... For a long time I was running 18" rims with 275/35's in back and I was getting 20mpg. Recently I went back to 16" stock rims from a 2002 E320 and I am now getting something like 28mpg. Did you check to make sure your tires are inflated properly? Seems like something easy to overlook, but I would notice a 3-4 mpg increase in fuel consumption if my tires were low on air.

Is your car consuming any oil? How do your diesel return lines look? Have you ran diesel purge through the injectors as of yet? What kind of oil are you using in the car? Do you notice a decrease in fuel consumption after an oil change? Have you pullerd your vacuum line from the Boost Control module to see if its clogged? To me it sounds like there is something major wrong, or a bunch of little things that are wrong here. It could be as simple as underinflated tires, and the wrong oil....... One more thing is the car lowered?

http://www.csgnetwork.com/tireinfo4calc.html

This calculator will tell you how much to correct your readings for different tire sizes.

So I am running 265/40/18 in the rear, and the car is lowered. Tire pressure should be fine, but what do you guys recommend?

I will check the other things you mentioned this weekend. As I mentioned before , the car seems to accelerate hard and fast - so I would be surprised if there is something wrong that is causing these the low mpg, I am leaning towards the fuel at the IP right now. I'll get that leak fixed this week, and then check the other things listed.

Thanks so much you guys.

ps - Husk, you went from 20 mpgs to 28 by changing tires?

yup, going back to 16's increased my mpg's by 8mpg, I was getting around 400 miles on a tank with 18's, now I get around 550 miles on a tank of diesel. It makes sense to go back to 16's with diesel at 4.50 a gallon.