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Discussion Starter #1 (Edited)
Hello Everyone!

I drive a 5speed 325ci 2001
When I first got the car a year ago, i was averaging around 21mpg, half highway half city. I haven't needed to go out of the city too often in the past few months, but i noticed a significant drop in my MPG. (Averaging City driving ~12.5mpg, I live in San Francisco, but not the hilly area)

Before we continue, I did have a series of CEL's one after another:

p0171/174, This was fixed when i replaced my MAF sensor.
p0102, MAF sensor
p0334, Camshaft position Sensor replaced
p0036, bank 1 sensor 2 o2 sensor replaced.
As of right now, there are no pending and no CEL lights.
All of these codes are related to the intake system, which as i understand, is in direct relation with MPG?

Right before my p0036, my differentials went out on me, so i replaced my Diffs' with a 330i, which is a bigger gear ratio... 3.15 (mine), 3.38 (330i)


I do know that upgrading to a bigger gear ratio does lower your mpgs, but from the advertised 18mpg city, to 12.5 seems kinda drastic for even a .18 ratio difference from a bigger Differential..

I was using my on board computer in the beginning for the mpg readings (after resetting it by unplug/replugging battery), but i figured it may make mistakes, so I calculate my mpgs at the fill up: Trip Miles/gallons of Gas used. (always 91 octane).

I was wondering if anyone can share some experience with what can cause low MPG without triggering my CEL? I also wanted to entertain the idea that in theory the parts thats were replaced to fix the CEL could have fixed the CEL but might not have fixed the MPG loss? <- Is that possible?

Thank you for your time, whether it be you were bored on the app or otherwise!

P.S! When i noticed the loss of MPG, I made a challenge for myself to get back to the 15-18mpg range, no success! (I've been laying off the lead foot this month)
 

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Finally someone that is telling things how it is!

FYI, there are people here reporting they are getting 34 MPG with these cars, its just not going to happen.

The newest EPA ratings for your car, which were adjust after your car was produced so the window stick will say differently is 18/21/27.

21 combined would have been about correct assuming you were not sitting stopped in traffic a good portion of the time. 21 MPG combined may have been slightly optimistic depending on if you manually calculated the mileage or just relied on the OBC.

I totally disregard the OBC values and only use then as a reference, to see if there may be a drop in the overall number if I am not manually calculating mileage values.

So a few things that directly impact mileage that need to be checked.

1. Engine operating temperature, 96C is pretty much where these cars should run, if any lower, fuel economy will take a hit.

2. Need to look at Fuel Trims and see where they are running at both a warm idle and steady highway cruise.

3. MAF, if you replaced it, where did you buy it and how much did you pay for it? If you have the original one, put it back on the car for a test.

4. What did you do to correct all the codes you had previously.

5. Do you know the shape of the DISA?

6. VANOS seals may be a factor?

7. Do you think the exhaust is restricted? If your car has a muffler flap, is it open when the car is not running?

8. You may need to trend/fingerprint the O2 sensors as you may have lazy/sloppy O2 sensors and you will NEVER get a code for these. See this thread for an idea of what you can find - http://forum.e46fanatics.com/showthread.php?t=1041726&highlight=

9. Do you have the orignal wheel and tire size?

10. You did change the differential, this will impact things in more than one way. Engine RPM may be higher at cruise, but speedometer may also be off and this along can make your numbers look much better or worse depending on which way the speedometer is off.

Suggest you get your phone/GPS and figure out how much the speedo and odometer is off and make an adjustment/correction factor for calculating your fuel economy.

Also use Fuelly to monitor and track your fuel usage over about 10 tanks and then average and see what kind of fuel economy you are getting.
 

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2001 auto 160k mi. = 15mpg.

Car in great condition but only used for two 3mi. drives per day
Your car is rated 17/20/25 per the EPA - http://www.fueleconomy.gov/feg/Powe...1&make=BMW&model=325i Sport Wagon&srchtyp=ymm

The problem is your conditions are probably the absolute worst. 3 miles is not long enough for the engine to fully warm up and you likely have a number of stops/traffic lights for this drive so 15 MPG would be about as expected given your drive and driving conditions.

Another honest fuel economy statement IMHO.
 

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Yes, I knew the reason for my bad mileage.

Mine does get 25+mpg on long trips even at 90+mph!

I was just throwing it out there for the OP.

Cheers,
Paul
 

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Yes, I knew the reason for my bad mileage.

Mine does get 25+mpg on long trips even at 90+mph!

I was just throwing it out there for the OP.

Cheers,
Paul
I can see these cars slightly beating the EPA rating because the EPA testing is performed on a Dyno and the fuel cut off on coast down does not really get exploited and the VANOS and DISA play into things a bit more on the road, but rarely will anyone be able to do better than 10% of the best EPA figures. And that needs to be some chill driving to get anywhere close to 10% over the highest EPA figure.
 

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but rarely will anyone be able to do better than 10% of the best EPA figures. And that needs to be some chill driving to get anywhere close to 10% over the highest EPA figure.
Don't know what chill driving, is but paying attention to the vacuum MPG gage and subtle gas pedal input will get you excellent MPG on the freeway at well over legal speeds.
 

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Don't know what chill driving, is but paying attention to the vacuum MPG gage and subtle gas pedal input will get you excellent MPG on the freeway at well over legal speeds.
Chill driving = what you are doing, light throttle application, non aggressive maneuvering but on the coastal sides of the US it is hard to find traffic light enough that are not constantly jockeying for a place to sit back and just cruise like chill highway driving should be.
 
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