Help, I’m looking for an answer/cause
I have a 2020 Kia Forte.
For 2 years, I had all my oil changes and service done at the same dealership with no changes to the car mileage.
The last oil change I had done with them, I brought it in with the mileage reading 42mpg and left with it dropping immediately from 42 to about 32mpg on the way home…
When bringing it back with my complaint, they denied anything different was done and it was the oil that did it.
Strange is that it was the same oil I always had them use.
I questioned that if this were the case, it would have changed along time ago.
It’s been more than a year and the 42mpg has never returned, even after taking it to a different dealer for oil changes.
Something more than just an oil change was done to the car.
Coincidence is that for some reason the oil change took a lot longer than normal that day.
I was happy that my car was getting much better mileage than normal.
Would even get 45-46 sometimes on the freeway.
Hoping that after the mechanic saw this he didn’t exchange a part thinking that I wouldn’t notice.
Anyone have an idea on what the mechanic might have done or replaced to make such a drastic mileage difference.
Thanks ahead of time.
I presume you have a miles per gallon real time display. No Kia experience, but here’s a few guesses
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Measured gasoline flow has become less accurate. In other words you may be getting the same mpg, just the display is wrong. Try measuring your mpg the traditional way. Number of miles covered since last fill-up divided by number of gallons.
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Coolant temp sensor not registering correct temperature. Ask shop to check it with their scan tool. If computer thinks coolant temp is lower than it actually is, it will inject too much gasoline. Ask shop to measure fuel trims.
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Brakes are dragging. Assess temperature of wheels after longish drive not requiring much braking. They shouldn’t be very hot.
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Driving style/situation has changed. If you drive shorter trips, more trips on surface roads vs freeway than before, or if ambient temperature is colder than before, those will reduce mpg.
Your car is rated at 31 city and 41 highway . If you are not checking with pen and paper you really don’t know what you are getting . The dealer did nothing to cause your readout . The dash readout can be as much as 3 miles per gallon off .
One of our vehicles aways shows more mpg in the town driving than it actually got . On the highway for long drives the readout is very accurate. You should reset every fill up and check it next fill up to see your difference.
So my question is how many less miles per tank of gas are you getting???
According to Google you have a 14 gallon tank, loosing 10 MPG per your OP, that means you should be getting 140 miles less milage on a tank of gas… If you were getting 500 miles out of tank of gas before hand then you have dropped down to 360 miles on a tank of gas… Is that what is happening??? Or are you just looking at the readout???
We have only a bit of info to work with here and my suggestion may be off base, but here it is. It sounds like you have been monitoring your “MPG” using the vehicle’s in-dash display. If that is true and you usually leave it alone or zero it when you refill, read on. If not, Save your time. First off, we assume you are looking at the dash’s MPG AVERAGE in either Trip Odometer A or B. If you are looking at the live MPG readout some cars have, just stop doing so. It is meaningless. If you have been using the dash display AVERAGE and the number has changed following a visit to the dealer, the chances are the dealer zeroed the MPG AVERAGE. Or they switched your view from Trip A to Trip B or vice versa. It is now counting up to a new average and your trip home is one that has a lower MPG than the total of your driving in all circumstances. I do not mean to insult your intelligence but wanted to offer this info. I test cars and can play the driver info display like a fiddle after years of practice. I use them to look at MPG when I test cars, but the folks above are right. These things are usually only accurate to about 5 to 10% and if you want your real mileage you need to measure gallons put in and miles travelled. Good luck! ps - The reason the dealer may have accidentally changed your display is that the maintenance reminders in almost all cars are set via the same display. So when they reset the maintenance reminders, they sometimes leave you in a different view, or they zero all the data (accidentally). Also, if the battery was disconnected for any reason that will also sometimes happen, but I think you would have noticed more resets if that occurred. Like all your audio presets.
The fuel mileage readout on your dashboard can probably be set to more than one mode, e.g. miles per gallon, kilometers per liter, liters per 100 kilometers, gallons per mile, etc. It may have been reset, as Goreham J suggests. And its accuracy may be off by a significant and variable percentage, as all suggest.
What does your owners manual say about this feature? If you set it to your preference, what does it say in the days and weeks to come?
Many of us will be getting better gas mileage in the next few months, with warmer temperatures and highway travel becoming a larger percentage of our overall travel distances.
To add a few more things to the excellent suggestions from George_San_Jose1:
- Wheel alignment problem. No code for this!
- Oxygen sensor problem letting the engine run too rich. Will not generate a code in the early stages in some cases.
- Fault that has caused the timing advance to go to fail safe (knock sensor fault). Will generate a code.
- Leaking fuel pressure regulator. Leaking fuel in to vacuum system. Common issue with old vehicles. Will generate a mixture too rich code as it becomes more severe and it will cause rough idle.
- Problem with the variable valve timing. Should have a code.
- Transmission lock up problem or transmission not shifting to higher gears. Won’t always generate a code that a basic emissions only scan tool will see.
What does happen when the wrong oil is used an the variable valve timing doesn’t work? Many VVTs depend on a certain viscosity of oil. Does fuel economy drop significantly?
I reset computer and get 24mpg on 100 mile route.
Wifey drives car for 3 weeks. Computer says avg is 19 mpg now.
I reset it and drive 124 miles and get 24 mpg.
1 month later avg is back to 19 mpg.
All is well.
I don’t think that there is anything wrong with the OP’s Kia. The dealer’s service tech probably let the engine idle after the oil change while he checked for leaks. This is what the tech is supposed to do.
+1
Additionally, most service departments keep engines idling for extended periods of time while waiting to jockey vehicles around their facility.
These posts that are about the dash fuel readouts are always interesting . We had one person post here that thought the average mpg reading was accurate . Another posted here complaining about their mpg and it turned out they were reading the dash wrong and did not know their was several things that it showed.
Maybe it can show MPG and tire PSI, etc., etc
Until someone reads the owners manual we are all just guessing.
Ya, the average mpg and instant mpg is confusing.
Oil change tech might reset oil life. He might not. He might not reset mpg info as well.
Confusing ? Are you making a joke ?
All (well most anyway) these newer gadgets on vehicles has done nothing but make people lazy… I use 1 trip meter for my oil changes and the other for gas, I reset after every oil change and or gas fill up… That is the easiest way to track your mileage… Then see how much fuel it took to fill up and how many miles you went and use a Calculator… Last time I checked the easiest way to calculate your gas mileage is to simply divide the number of miles traveled by the number of gallons of gas your vehicle took to refill . In sum, that’s miles driven divided by gallons of gas used…
My daughters car has all kinds of safety crap and other gadgets on it and she doesn’t use any of them except for the auto bright light dimmer that took her a long while to even realize it had, due to a sign reflecting just right and it dimmed her lights… And she is a excellent driver and can wip in a parking spot so fast it will scare you and she is perfectly centered and straight… But I also started letting/teaching her how to drive at 13 to 14 years old in safe places only… when she took her driving test the instructor came up to me and expressed how good she did, didn’t do that to anyone else in the room while we were there…
How ever I do enjoy driving her car with the LDWS on the back roads really fast to see how it goes crazy carving out the curves… hehe
But then again, I do see how a lot of people drive and they need all the help they can get… 
For those who don’t know - LDWS ( Lane Departure Warning System )
And for you Luddites many of those safety features can be of benefit.
For those who don’t know Luddites is often used to refer to anyone who is opposed to new technology, particularly in the field of IT
My fuel economy was at 35 MPG but now it has dropped to 28 PSI! I can see that happening. In the 80s people would come in with Ford vehicles complaining that something was wrong because the oil pressure gage would movo. So they would replace it an on/off switch so the gage was always in the same spot.
PSI, like MPG, goes down in cold temperatures. Both are likely to go higher in the next couple of months!