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2013 Hyundai Sonata MPG went down after replacing Battery and Plugs

Hello All - This is my first time posting on this site. I have been a fan for CarTalk for many years.

I have a 2013 Hyundai Sonata GLS 2.4L Engine and recently replaced the Battery and Spark Plugs myself. My car has 55,000 miles and I have had all the normal maintenance performed. The maintenance guide suggested the plugs be replaced at 45,000 miles I waited until 55,000 as I was not having any problems. The last couple of trips to auto shop showed the battery slowing losing the ability to retain a charge so it thought might as well replace now rather than later.

After changing the battery and plugs I went to fill up my Car with Gas and the MPG dropped. Normally when I fill up the car with gas the dashboard of my car would show 420 miles until empty. Recently when I filled up the car it showed 370 miles until empty. I am thinking this is due either to replacing the battery and having the computer reset or I set the gap on one or more plugs wrong. The car is not misfiring so I am thinking it is the replacement of the battery and resetting the computer. I was going to run the care for a couple of tanks and see if the MGP change. I am also going to monitor the mileage and see if it matches the overall MPG.

Is my thinking about the battery and computer resetting correct? Any other thoughts.
Thanks in advance for any and all replies.
Bob B.

The “miles to empty” estimator works by factoring in what kind of mileage you have been getting lately. If you replaced the battery, and then drove directly to a nearby gas station to fill up, then the only information the computer has to go on is “mileage is absolutely terrible” because short trips where the car barely even warms up get much worse mileage than normal trips do.

For future reference, the only remotely accurate way for you to figure out what kind of mileage your car is getting is to divide the number of miles you drove since the last fillup by the number of gallons you put in on this fillup. So, 300 miles, 10 gallons, you’re getting 30mpg.

Any other method (at least, any that you can get your hands on) of determining mileage is too inaccurate to be of any use whatsoever.

I agree with that computer reset theory. Since the reset , it has very little input from which to average.

You should see the MTE on my Expedition !
Using the input from around town, it will spit out numbers based on that when . . . I fill up completely for a day trip to Albuquerque . . set out on I-40 for the 140 mile ( one way ) trip . . As I look at the MTE . . and look at the mile posts. . I can clearly see the evidence of its averaging calculations.

    • 480 mte… . mile post 26 . . mile post 27 . . mile post 28 . . 479 mte !
      And so on for MOST of the trip ! Even on the return trip , it seems that I can still drive at least 1-1/2 TO 2 miles before the mte subtracts just one.
      It has so many city miles in its history that it really has no clue when I set out on the rare hiway trip.

It took a road trip to Disneyland last year for it to catch up with itself.

Yep, not a problem because the computer reset. You did use the exact same make/part number of plugs that came out, right?

I just checked the owner manual in my 2010 Kia Forte SX 2.4L. “Spark plugs. Replace 105,000 miles or 10 years.” My battery tested weak (390 cc amps) so I had it replaced at 39,000 miles/6 years. The only thing I discovered “lost” was radio pre-sets. Since I only use 2 it was no problem and 2 minutes to reset. I normally drive with the display in average MPG. I know mine automatically resets when I have the tank filled. The first time I filled up at a station on the highway and had to accelerate briskly to highway speed it was reading 3.6 MPG!!! After 10 miles highway cruising it was 30+ MPG. The next time I am in that situation I will try MTE. I expect it will initially be dismal. Next service I will check with the dealer concerning the 45,000 mile spark plug change. It could be a TSB that did not trigger their computer at 39,000 miles. I’m sure by now your MPG/MTE is back to normal. When I first got my car I manually checked MPG a few times and it was so close to the display I haven’t bothered since.

Thanks for the replies and confirming my idea.