sudden 33% mileage drop

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Gotta surprising drop in mileage – guessing @ 33%. Is this a cold weather thing?
(1994 Nissan Quest; 6 cyl, 289Kmiles; no leaks; engine sounds good…)


  1. Before E10 was the norm, I drove across country from Utah to Indiana pulling a loaded trailer in my 99 Explorer. I averaged 13 mpg at every fill up, even going over the Rockies in Colorado; however, in Kansas, where all the stations were selling E10, my mileage dropped to 8 mpg and the power to pull dropped way down (remember, this is with a heavy load). It went back up to 13 mpg after the first fillup in Missouri.

    When not heavily loaded, my Explorer averaged 17.5-20 mpg before E10 was normal. Ever since E10 became the new normal, my mileage has been 14.5-17 mpg. So, unloaded, I get the expected 3 mpg drop. When loaded that drop becomes 5 mpg.

    My owners manual states that E10 will not hurt the vehicle, but also advises the owner to expect a drop in fuel efficiency and engine power.

  2. I and everybody I know have noticed a dramatic drop in MPG for the last few months. This has been in TN and to a lesser degree in TX. Local gasoline formulations may account for some of the variance. I would be tempted to check out ethanol free gas if you can get it in your area.

  3. my mileage dropped a large amount when one of my cylinders lost compression due to a burnt valve. do a compression check on all your cylinders.

  4. Hi Steve,

    A 33 percent drop in mileage is not normal! Ethanol-laced gas will typically give you 2-3 fewer MPGs, relative to 100 percent gas.

    Something else is wrong here. The engine is very seriously out of tune (or needs major work) or the transmission is slipping, or something else like that.

    Is the MIL light on? If it is, you may have a plugged-up cat, which would not be unusual given the mileage. A major increase in exhaust backpressure could result in the mileage drop you’ve noticed.

    • Thanks for the advise, Eric!

      I’ll pursue these remedies as you’ve recommended. Fortunately, I do have an excellent mechanic.

      (Some notes per your reply:
      1. GAS I do run the worthless eth-gas – I’ve found real gas nearby – so I’ll change, now.
      2. TUNE the engine doesn’t appear rough running – no colored smoke exhaust.
      3. TRANNY doesn’t seem to be slipping.
      4. MIL light not on.)

      I really appreciate your sensible guidance and reality checks.

      Thanks so much!


      • You bet, Steve!

        The E10 (regular unleaded with 10 percent ethanol; the “gas” sold in most areas) would not account for such a dramatic drop in mileage. However, E85 (85 percent ethanol) might. It occurred to me that you may have accidentally filled up with E85. Is there any chance you did that?

        If not, your mechanic will just need to do some diagnostic work, as mentioned in previous posts.

        * Establish whether the engine is mechanically sound (compression check/leak down check, etc.)
        * Check state of tune of engine (fuel/ignition/emissions system, including cats).
        * Check the rest of the driveline.

        It’s in there somewhere!


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