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Fuel economy

5K views 39 replies 16 participants last post by  jfoj 
#1 ·
I've been reading around and it seems to me that other people on the forum are getting their mpg in the mid to high 20's. I have an 05 325i and in the city I average around 15-16 and highway 18-19. Why is this? I'm a "spirited" driver but its not like I thrash it, so what could be causing such low mpg?
 
#7 ·
What elevation are you at in Colorado?

Might be Winter fuel blends?

First change gas station and blend and see what values you get by manually calculating mileage.

Make sure your thermostat is good, see 3rd link in my signature, display coolant temp on OBC display, should be around 95C.

DISA and VANOS seals play into mileage as well.

For DISA info, see first link in my signature.
 
#10 ·
On the highway, you should be somewhere around 35 mpg at an avg speed of around 70. I get 34.5 in fifth...ymmv slightly...and should be slightly better.

Start with thermostat...bet you're showing a little to the left of dead center on a cruise. Shows it's broken open and will rob mpg.

Fuel filter also will take mpgs.

Then other basics, maybe ignored...air filter, plugs (at 100K miles), and then, there's the odd malfunction that seems to hold the brakes on...e-brake sometimes binds...(some odd ball things like this happen rarely, but they happen).

Start with basic tune up stuff and go from there. Get codes read too...oh, and you could have an air leak too...almost everyone does...and when the car sees it's running lean, it adds fuel...lower intake boot is one common leak...but check as many vac lines as you can...look beneath cabin air filter housing behind intake manifold.

see jfoj's sig
 
#13 ·
oh, and you could have an air leak too...almost everyone does...
So true in many ways!

:lmao:

Need to add this one to your standup routine!
 
#16 ·
I live in Albuquerque and we are at.5200' elevation. I have two E46s one is an automatic 02 325i average mileage in mixed driving is 22 mpgs. My other is car an 04 325ci 5-speed and I have been averaging 24 mixed driving. On a recent trip to Denver I averaged 29mpg going and coming back 31 mpg with my 325ci. I've also notice my mileage is better in the summer and I always use 88-89 octane. Sometimes I use 91 octane and I have noticed 1 mpg improvement. All mileage is calculated and every fillup is recorded. I have installed KN filters an maintain tire air pressure. I probably would get better mileage on my automatic if I remove the roof rack.
 
#17 ·
OP, I get 21mpg mixed(more like city than highway).
I have to drive from DU to Anschutz Medical and back 2 times a day at 8 30am and 5 pm.
You can imagine i25 then 225 or Colorado blvd at those times.
If I take it for a highway run, to Georgetown or Idaho springs to fish I get around 28mpg.

I use shell 91 mostly.

Your car might need a little tune up.

PS: I drive an 01 auto 325.
Shoot me a PM. I am here in Denver, there may be a Denver meet up soon.
 
#20 ·
The real question is what is the coolant temperature?

These thermostats fail open ALL the time and the engine can run too cold, when this happens, fuel economy is the first thing to take a dive.

95C is about typical, maybe slightly higher.

Hidden OBC will answer this question in a single trip.
 
#33 ·
20 minutes is plenty of time for the engine to fully warm up. A side effect of a bad stat is also slow and long warm up times. Your car should likely be fully warmed up within about 10 minutes of driving.

Side effects are poor fuel economy and extra mayonnaise in the crankcase. What happens the oil take a lot longer to warm up as compared to the coolant. So you risk not cooking water vapor out of the crankcase as well as the CCV possibly freezing up and possibly sucking the oil out of the crankcase.

You may also risk some accelerated engine wear due to slow warm up. I believe the typical operational temp should be close to 95C as I recall? I do not drive the E46 often, so I do not recall what the typical temps run. I will try to check my E46 sometime this week to confirm.
 
#35 ·
Last tank I got about 16 mpg. 250 miles for a tank of about 15.4 gal. No check engine lights are on, no misfires. Used PA Soft and did not pull any codes. I do mostly city driving with a fair amount of stop and go, but I am not slamming on the throttle and I do some highway driving. I have a stick shift 323i. The temperature gauge gets right to the middle when the car warms up. I notice that the mpg indicator flies down to 12 mpg with the slightest application of throttle. For example if I am coming out of a rolling stop and am in first or second gear, it will hit 12 mpg even if I slowly accelerate to 30 mph. Additionally, if I am going down a gradient, such a parking garage and the car is in gear and I am applying a small amount of throttle, the mpg indicator flies to 12 mpg. Any thoughts?
 
#36 · (Edited)
Any thoughts?
Yes, consider the mpg gauge entertainment, not something to seriously use to check mpg. Every car will plunge to 12 mpg with a slight bit of acceleration. It's only the preponderance of light cruising that allows us to average mileage up in the mid twenties or better.
If you want to get serious about improving mpg, first get serious about documenting things. Write down your odometer reading and take the reading from the pump in gallons when filling to do your calculations.

Next start going thru the list of normal things that can affect mpg to bring your engine up to standard. Air leaks, O2 sensors in good shape?, MAF good, etc. for instance.
 
#37 ·
For my calculations from the last couple of tanks I reset the odometer after filling up and when I refilled I divided the miles travelled by the amount of petrol I put in. I guess I could check for air leaks, I don't know how to check for o2 sensors, but they were changes within the last 20k. Again no codes or shadow codes on the pasoft. It just seems like the car is a little sluggish compared to my turbo audi which gets about 22-23 mpg. Perhaps thats because there are less hp in the e46, but averaging 250 mile range per tank or 15-16 mpg seems a bit low.
 
#38 ·
The temp gauge in these cars is also for entertainment to some extent.

The gauge will stay at 12 o'clock over a 50F range!!!!

You need to access the Hidden OBC menu, 3rd link in my signature, and display the coolant temp on your OBC display. Normal temp is 90-95C, anything lower in town or at idle is a bad stat.

DISA not working also impacts mileage a bit was well, first link in my signature. VANOS seals leaking can also impact performance and mileage as well.

But the first thing to do it check coolant temp, then change gas station and brand. What EVERYONE needs to realize is the issue if more than just the fuel mixture, it is also a function of how well the pump is calibrated as well. You NEED to pay attention to the state calibration stickers on the pumps. If the date is more than 1 year old, I would be checking for a station with newer calibration stamps.

Winter blend fuels are also and Ethanol percentages are also a factor. FYI there is a push to consider going to 15% Ethanol, not sure it has been approved yet, but Ethanol blend percentage is not closely regulated and you would be surprised what the percentage you actually end up with may be!!
 
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