Sudden drop in gas mileage 2017 Kia Sedona 36+k miles



I have a 2017 Kia Sedona. It is maintained to the owner’s manual. I always do the oil changes at Big-O-Tires, same shop. On 25 July, 2021 at 35562 mileage, the tech called to say that the Drain Plug was stuck and I had to replace the pan ($300). I pushed back. I showed that they had done the prior oil-change so they should be owning the problem. After a day of going back-n-forth, they finally owned up and said “Drain Plug had debris” and they replaced the drain plug but not the oil-pan. I had a non-big-o-tire Master-Tech then check that there was no apparent oil leak and I moved on. NOTE: The prior oil change, due to covid reducing the car usage, was at 31805 miles on Dec 15, 2019 (3757 miles and ~1.5 years earlier).

ISSUE: I track gas filled in my cars religiously in an Excel file. Since Mar 2021 to Jul 22, 2021 my mileage was 17.79 MPG (1299 miles) while after July 31, 2021 to Sep 13, 2021 we drove 403 miles during which it has step-function dropped as shown in the table below to 14.43 MPG (by 19%) and is seen at each gas-fill.

Date Miles / Gal
On JULY 25: Oil Change issue describe above…

***QUESTION: Could the reason for 19% step function drop in the mileage I get be anything related to the oil-change issue described above? If so, how and your suggestion on how should I proceed… ***

Relevant information:

  1. We have not seen such a drop in mileage for any of our cars in the last decade.
  2. We are two drivers in the household but I am consistently the primary driver on this car before and after the oil-change.
  3. The car has not change its typical routes i.e. from highway to city.
  4. There is no alarm for tire pressure being off and I check the tire pressure periodically. I can check again tomorrow.

Please comment on the question above.


I bet you’re supposed to change your oil more frequently then every 1.5 years.


Based on the detail provided, I’d guess the wrong oil weight was used. There is no way to verify this other than to change it again to the correct weight and note the mileage.

My other thought is that, purely coincidently, the coolont thermostat has failed and not allowing the engine to warm up quickly. Watching the temp gauge should confirm or deny that. The gauge should rise to the middle of the range within a mile or 2.


Thanks. I will watch the temp gauge and update here. If things don’t work; I will get another oil-change done quickly and update here then… Thanks!
If there are additional inputs, I welcome those too.

another idea to entertain: are you driving the similar route and/or distance as before?

if (due to COVID) the range per drive-cycle was reduced, it might explain the MPG drop

In addition to the good ideas above, another possibility, you may have a stuck injector. The fuel cleans the injectors as it passes through , so this could be caused by not driving enough. If so, an OBD II scan might show diagnostic codes indicating overly rich operation, or you may notice the engine cranks for a longer period of time than it did when the mpg was normal.

Coincidentally, I’m having what I think is a similar problem, overly rich exhaust, related to not driving the car enough, due to Covid.

Did you just replace tires?

Not only do OE tires have better rolling resistance, but new tires have worse RR than worn tires.

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Thanks for all the additional ideas.

  1. The tires have been the same.
  2. Stuck injector is a possiblity.
  3. I thought long and hard, I am guessing the following is the contributing factor. A little funny/odd but certainly a likely cause: "My son turned one around the same time I did the oil change. As he turned one, he started showing more interest in watching airplanes take off and land. So, I started going to a local airport to show him the planes. That was idling with AC on for 30-60 minutes each time.
  4. Tire pressure: Today, I found out that Big-O-Tire filled the air in my tires at 30 psi while the spec is 35 psi. Low tire pressure is known to reduce gas mileage.
    After a high high-way round-trip of ~250 miles, of which 50% was at 30 psi and the remaining at ~36psi, the mileage is improved.
    I will continue to monitor and report updates. Thanks!
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That is when the gasoline was wasted.

Was that documented on the invoice?
Your tires are cooler today than they were in July, therefore the pressure is lower, you need to add air to the tires as the weather cools.

Invest in a good tire gauge and a means to add air, like a 12V portable inflator.
Just takes a few minutes to check and top off in the morning once a month.


In addition to slightly improving gas mileage, tires that are properly inflated will yield safer handling/road-holding, as well as longer tread life.

I really hope that the OP inflates his tires to at least the mfr’s specified inflation pressure very soon, because if they were under-inflated by 5 PSI a few months ago, a recent drop in ambient temperatures could have easily caused those tires to be dangerously under-inflated by now. I inflate my tires to 2 or 3 PSI over the vehicle mfr’s specifications, simply to provide a little bit of an “edge” when temperatures drop.

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Me thinks that is a Clue to reduced miles per gallon .