Why Did the EPA Lower The 328i\’s MPG Rating? We Have An Idea

For the first time in recent memory (if ever) the EPA shattered BMW ‘s Monroney dreams. Taking the 8 speed automatic 328i ‘s lofty 36mpg highway and relegating it to a mundane 33 mpg. What that means is that the 4 cylinder N20 that is newer and is marketed as the more efficient engine offering is rated the same as the larger N55 inline six cylinder in the 335i. How can that be possible?

The first thing that came to mind is that the test was botched, BMW may have used a different test protocol or the EPA tested with a car that had some issues. One mile per gallon adjustments are typical but a minus three is unheard of, so something is up. As we dug deeper we discovered something that wasn ‘t advertised by BMW and something that was a significant departure from the normal BMW mantra. We were utterly surprised. We had discovered something that even some BMW employees weren ‘t aware of or even trained on. The 328i and its N20 are stated to run not on the normal BMW recommended minimum 91 octane BUT 89 octane. (We may be living under a rock but we are not aware of any other BMW that states 89)

Owners Manual:

Fuel Vehicle equipment All standard, country-specific and optional equipment that is offered in the model series is described in this chapter. Therefore, equipment is also described that is not available in a vehicle, e. g., because of the selected optional equip‐ ment or country variant. This also applies for safety-related functions and systems. Fuel quality Fuels that are marked on the gas pump as con‐ taining metal must not be used. Refuel only with unleaded gasoline with‐ out metallic additives. Do not refuel with any leaded gasoline or gaso‐ line with metallic additives, e. g. manganese or iron, or permanent damage to the catalytic con‐ verter and other components.◀ Do not refuel with ethanol Do not refuel with E85, i.e., fuel with an ethanol content of 85 %, or with Flex Fuel, as this would damage the engine and fuel supply sys‐ tem.◀ Required fuel Gasoline with lower AKI The minimum AKI Rating is 89. If you use gasoline with this minimum AKI Rat‐ ing, the engine may produce knocking sounds when starting at high outside temperatures. This has no effect on the engine life. Minimum fuel grade Do not use any gasoline below the mini‐ mum specified fuel grade; otherwise, engine damage may occur.◀ Use high-quality brands Field experience has indicated significant differ‐ ences in fuel quality: volatility, composition, ad‐ ditives, etc., among gasolines offered for sale in the United States and Canada. Fuels containing up to and including 10 % etha‐ nol or other oxygenates with up to 2.8 % oxygen by weight, that is, 15 % MTBE or 3 % methanol plus an equivalent amount of co-solvent, will not void the applicable warranties with respect to defects in materials or workmanship. Minimum fuel grade The use of poor-quality fuels may result in driveability, starting and stalling problems es‐ pecially under certain environmental conditions such as high ambient temperature and high al‐ titude. Should you encounter driveability problems which you suspect could be related to the fuel you are using, we recommend that you respond by switching to a recognized high-quality brand such as gasoline that is advertised as Top Tier Detergent Gasoline. Failure to comply with these recommendations may result in unscheduled maintenance.◀ pg 262 328 manual Online Edition for Part no. 01 40 2 608 633 – 11 09 490

This is something that could greatly influence economy numbers. We may be going out on a limb here and saying that BMW did its internal testing with 91 and the government used 89 in their test, but this is possible and likely what happened. BMWNA does its testing prior to the owner ‘s manual and stickers for the gas lid are published so if BMWAG had made this decision and the engineers (at BMWNA) were not properly informed of the decision to use 89 it could easily happen especially when everything else is tested with 91.

Will we ever find out what really happened with this? More than likely not, as 2012 is a short run for the new F30 3 Series and adjustments will be made for the model year change over to 2013 in June. This is only a theory of ours but it is one of the only scenarios that makes any sense.

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  • Cameron Love

    I wonder if the 528i, Z4 sDrive 28i, and the 2013 X1 and X3 are going to have the same issue…

    • BimmerFile_Michael

      I believe they still state 91 as the recommended minimum octane.

      • Cameron Love

        I’ll check at work tomorrow…this was a big thing at our dealership when the EPA dropped the MPG to 335i numbers…

      • Ellinara

        My e36 states min 89 aki but when i started to use 91 octane i noticed significantly better engine response and mileage

    • I have a Z4 with the N20 on order and just re-read the manual, it clearly says 89 octane MINIMUM and 91 recommended… 

  • Chad

    they could just put out a recall on all of the F30 owner’s manuals…

  • bc works

    There is normally a difference between recommended and minimum. I have not seen BMW use the two terms together. I think most BMWs have a recommended 91. My N52 also has a ‘highly’ recommended 91, but the minimum is only 87. Adding a turbo to make a N55 and only increasing the minimum to 89 seems inadequate, but given previous fuel quality issues, maybe they felt that is the highest minimum they could require. The lack of a recommended AKI and only specifying the minimum looks more like a publishing error than a shift in design standards to me.

    Decreasing the octane from 91 to 89 in an engine designed for 89 should not make a 3mpg difference. Both octanes have the same specific energy. The higher octane merely has a greater resistance to pre-ignition, it does not burn more efficiently or with more energy. I think the rationale for the drop lies elsewhere.

    • BimmerFile_Michael

      You are forgetting the Engine management decreases the timing thus reducing power and in the end it does decrease MPG. I have tried using 89 myself and it was a 2 mpg decrease in a MINI.

      • Cameron Love

        It kills my MINI!

  • Johnparke

    That actually is a brilliant thought. I bet BMW didn’t see that one biting them in the ass though.

  • Good theory.  If that engine is as finely calibrated as BMWs seem to be, I can see octane making a difference on a BMW 4cyl turbo.

    My car is rated at 27 highway, but it is not unusual to get 35 highway at 75mph.   I got 36mpg round trip last night.  Since I don’t really trust that MPG computer in the car, I once drove 500miles on a tank just to see if I could.  Not really sure why I am getting 8mph greater (or 5mpg greater city) than what is on the sticker, but I’m not complaining.   Car:  2008 e91 (3.0 6cyl) with 93 octane. 

  • Carzzi

    My 1994/95 E34 530i MB60B30 V8’s recommended rating is also 89 AKI. that surprised me, because all my other BMW’s were rated at 91/93 (M20, M52, S52, S62).