BMW 325i Mileage

According to my maintenance manual, my 2003 325i should get 18 MPG city and 27 MPG highway. I get 17 MPG no matter if I am doing city OR highway driving. And I commute (highway) 74 miles per day. What possible solutions should I discuss with my mechanic?

Check the air in your tires. I can’t imagine how you can get the same mileage in both city and highway mileage unless it a combination of both. Low tire pressure will kill your gas mileage. I run 32 psi in everything I own and always get good mileage and tire wear. Check your owners manual or vehicle placard for the correct pressure. Never use the pressure listed on the tire. You might want to have your fuel system checked out if the tire pressure is ok. Leaking injectors can also affect fuel economy.

Is your highway commute stop and go? That’ll drop mpgs. 17 is still low, though, those 325i’s are known for pretty good mileage. If tires are OK (check them cold, first thing in the morning, with a good gauge), then you might want a good BMW mechanic to give it a thorough going-over. How long have you owned it? Has the mileage gotten worse?

How many miles on your Beemer?? A critical part called the “Oxygen sensor” is what controls the fuel mixture and plays a large part in determining mileage. They have an expected life of 100K miles…Like your spark plugs, they need to be changed in order to maintain proper engine operation…Your mechanic can scan your ECM and check that the timing is being set and adjusted properly, another important factor in achieving good mileage…

I had the tires checked and they were low; however this didn’t help with the mileage problem. However, I think your idea about having the fuel system checked is a good idea. Should this include the fuel injection system too?

I really appreciate your response.

Very little stop and go. I would say about 68 of those miles are highway. Tires are okay - I had them checked. I bought the car used about 10 months ago. And yes, the mileage went from 18-19 down to 17. I used to have a 525i and I was NEVER in the gas station. I used to get about 28 MPG and thats why I can’t figure this out. Not that I know much about cars…

Thanks so much for your response!

About 99,000 Miles. This is a good suggestion. I never knew oxygen sensor’s had to be replaced.

Thanks so much for your help!

Yep, something’s wrong, you should easily match the 525i (assuming the 28 mpg was on the same route), and the 325i should do a bit better. No ‘check engine light’ or any other light illuminated?

Absolutely. Sorry I did not make that clear.

They don’t unless they fail. But I agree with Caddyman’s suggestion to start by having the computer scanned for any stored codes.

Post back with the results.

Can I ask how you’re determining your gas mileage?

Consumption shows on all 325 dash computers. All of them I have checked by dividing the miles driven by the gallons consumed were quite accurate. Hopefully Martha has checked hers that way too.

O2 sensors can slowly degrade and loose calibration. They still work, but the voltage signal is a little off so the fuel mixture is also a little off…They sit in a high velocity, high temperature exhaust stream and the platinum catalyst slowly gets eroded off the ceramic core…

The Check Engine Light does illuminate when the key is turned on doesn’t it?

I would think if this engine were legitimately doing that bad on mileage the CEL would be on: assuming that this used car has not suffered a CEL bulb removal operation.

Drop by a local parts store like AutoZone, Advance, or whoever is local and have them scan the car for codes. It’s free and only takes a few minutes.

OK. Thanks for the advise!

Manually - filling the tank to capacity and keeping track of the mileage.

I don’t have any “consumption” light on my dash. I have a feeling some lights were disconnnected or removed. I’ll have to read my manual. Female LOL

How fast do you drive? If you are in top gear, try to keep the RPMs at or below 2000. If you are going 75 in a 55 zone, as many people do, it will cost you in gas mileage.


It’s likely your car’s shift point to 5th gear is around 50 MPH. If you run at 2000 RPM in 5th at 60 MPH, you will be way higher at 80 MPH. It seems to me that your speed is likely why your mileage is so low.