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Discussion Starter #1 (Edited)
I'm a bit disappointed in this loaner C300 that I'd been driving for a week (returned it today). It a MY10 with about 14K on the ODO. In that time, I struggled to hit 24mpg on highway driving - I had a 3hr commute to the DC area. I finally pegged 24.7mpg after driving 65mph for more than an hour. Is this everyone's experience? I was xpecting high 20s to low 30s. Is it 'cause motor may not have been properly broken in?

I'm particularly unimpressed because that's similar mileage my 01 E430 (8 cylinders) does on same trip, driven much harder. My 96 Nissan TwinTurbo does slightly better. What gives?
 

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2000 Designo SLK-05 C230WK-05 E320CDi-2010 GLK 350
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I'm a bit disappointed in this loaner C300 that I'd been driving for a week (returned it today). It a MY10 with about 14K on the ODO. In that time, I struggled to hit 24mpg on highway driving - I had a 3hr commute to the DC area. I finally pegged 24.7mpg after driving 65mph for more than an hour. Is this everyone's experience? I was xpecting high 20s to low 30s. Is it 'cause motor may not have been properly broken in?

I'm particularly unimpressed because that's similar mileage my 01 E430 (8 cylinders) does on same trip, driven much harder. My 96 Nissan TwinTurbo does slightly better. What gives?
The last guy that had it probably filled the tank with 87 octane regular before he turned it in to save a few bucks. The knock sensors de tune the engine so much by retarding the spark that it can lose 20 horsepower as well when low octane fuel trips the knock sensors.

If you try this on your own car, be sure to reset the ECU after you refuel with the proper stuff because it may take a lot of time for the system to learn the new driving parameters and you will be wasting gas while that is happening.
 

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2008 C350 Blk with Gray (sold), 2011 E550 Cab, 1986 Porsche 911 Carrera/Targa 2010 Mazda 3 Hatchback
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I drove from Atlanta to my house south of Daytona Beach, approximately 350 miles or so. I had the cruise set at 82+ for as much as possible and was getting a solid 28 MPG. For long hwy drives that has been my experience with this car which has a slightly bigger engine than the 300 you drove. On shorter trips, less than 100 miles or so the on board computer typically only gets up to around 25, it registers changes rather slowly. I also checked the old fashioned way on several trips and those numbers seem pretty accurate.
 

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Discussion Starter #4
That's more like what I was expecting though I was not doing more than 75 on most stretches. Could be bad gas like n5160u said, but when I got the car last Friday it had under a quarter tank. I drove that quarter tank to near vapor - not on purpose, but was just busy running around doing what I needed to do. I then filled it up with Premium from our local WAWA. So, by the time I headed down to the DC area, whatever bad gas was there should have disappeared. Anyway, I have no explanation for that low a mileage.
 

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2000 Designo SLK-05 C230WK-05 E320CDi-2010 GLK 350
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That's more like what I was expecting though I was not doing more than 75 on most stretches. Could be bad gas like n5160u said, but when I got the car last Friday it had under a quarter tank. I drove that quarter tank to near vapor - not on purpose, but was just busy running around doing what I needed to do. I then filled it up with Premium from our local WAWA. So, by the time I headed down to the DC area, whatever bad gas was there should have disappeared. Anyway, I have no explanation for that low a mileage.
Although the cars control systems are quick to de-tune the engine because of low octane fuel, they only gradually try settings for higher octane fuel to prevent engine damage from occurring. This process might take several refueling stops before the car is happy with the fuel grade.
Your best bet is to reset the ECU so everything including the adaptive transmission learned response to the bad fuel including its newly "adapted" shift points are reset to the default settings after an accidental run with low grade fuel.
 

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2013 C250
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I have averaged 22-23 mpg in mixed suburban driving over the last 3 years. Tanks with mostly freeway driving at 70-80mph, usually 27-28 mpg.
 

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Have 8k miles and since new, computer shows 21.3 MPG. This includes lots of over 110 MPH driving uphill from sea level to 10,000 feet, just to get to the next city. Very satisfied with the mileage, and this on Mexican gasoline.
 

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2008 C350 Blk with Gray (sold), 2011 E550 Cab, 1986 Porsche 911 Carrera/Targa 2010 Mazda 3 Hatchback
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Throw in a bottle of Cuervo and you might get 22+!(No worms)
 
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