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Discussion Starter #1
Why do I get great mileage on highway (~27 mpg) and only 19-20 around town? I think I was getting abut 23 mpg in the city a few years ago (but I drive around the city more than hwy now). I was just reminded of the difference when I went on a 270 mile round trip using only half a tank. This is the first all highway trip I have taken in this car for many years. So, I forget what my mileage was before, but I do recall getting over 400 miles in a tank before. I am used to refueling at about 320 miles. I am very light on the throttle around town and I live in a relatively flat part of the country. On my trip I averaged 70-75 mph with no a/c.

Before any naysayers jump in on tire size, I am running the right 195/65 15" tires. I have also lowered the car, but I would not expect to reduce drag that much. Since I am not using brakes routinely on the highway, I considered that my brakes were sticking around town and lowered the city mileage. This is not the case because the car still coasts with ease when I am parking. Just curious if the fuel management needs to be tweaked any for low speed driving.
 

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89 300E; 00 E320
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2,511 Posts
If you have a W124 with a Bosch KE3-Jetronic (CIS-E) fuel injection and it idles smooth when warm and has good response just off idle then you're probably ok. You can check your fuel mixture if you want and make sure that your duty cycle at idle is no more than 10 points away from duty cycle at 2500 RPM's.

If it is greater than 10 points off then lean the fuel mixture by adjusting counter clockwise 5 or 10 degrees at a time and recheck duty cycle at idle and at 2500 RPM's.

Adjust until it is within tolerance and engine runs smooth (if it is too lean, then it will not run smooth); duty cycle will be close to 50% when it is adjusted correctly.
 

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Discussion Starter #3
Not CIS, but rather HFM with MAF. Its a 94 24v 104 with about 132k miles on it. It seems to idle fine, but the exhaust smells a little unburned if I run it through the gears with some speed.
 

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1995 E320
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2,495 Posts
Couple things.


Winter fuel. You lose fuel efficiency.
Your mpgs should actually be a range; you never drive exactly the same, in exactly the same traffic, at exactly the same time, all the time.

Getting just below 20 in the city and high 20s sounds reasonable to me.
 

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1982 300D
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182 Posts
Question,
Why do I get great mileage on highway (~27 mpg) and only 19-20 around town?
meet answer:
I drive around the city more than hwy now
Unless you drive only in the city for a whole tank and then only on the highway for a whole tank, your efficiency numbers are a mix. Increase the percentage of city driving, and your overall mileage goes down.
 
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