Poor gas mileage after oil change?

[358R]

I'll be brief. Just recorded best mpg ever with me driving, 39.1 mpg. Then, 26.9 out of nowhere. No changes in highway/city mix, driving style, etc.

Things that happened in order of note:
-oil change on 10/5
-cleared old dtcs from several months back
-record 38.2 and 39.1 mpgs, sequentially
-filled up with gas and it stumbled as if i had tried to over fill the tank but i stopped at the first click as always
-yesterday noticed i was at 115 miles on a 1/2 tank. Usually closer to 200.
-cleared oil light
-recorded 26.9mpg

Other notes:
-AFR hovers around 14.7 under constant throttle, around 40 when cruising/shifting (no throttle), drops to 12.4 under heavy throttle
-seemingly no loss of power, boost seems normal for a tune, ~23psi
-oil may be slightly overfilled (can't see the word "max")... But two of my best mpgs ever with it overfilled?
-disassembled and reassembled intake to verify no vacuum leaks
-no smell coming from the brakes
-accidentally pressed one of the cruise control buttons and RPM gauge dropped to 1k for a second...but engine RPM did not change.

What am I missing? Ask me questions. Just a bad tank of gas? Something else?

 

[gtx3076]

Are we on the winter gas now? Summer gas returns higher mileage.

Nothing else sounds out of the ordinary.

 

[358R]

Are we on the winter gas now? Summer gas returns higher mileage.

Nothing else sounds out of the ordinary.

Is there really that big of a difference? Seems like the lowest i ever recorded was about 32mpg.

 

[Old Mike Emerson]

Your numbers look good, about the same as mine, my mileage is only 24.9 but I've been beating on her a little bit, Lil Badass loves it and I have a Dizzy tune. I'm going to be easy on her after next fill up to see what mileage I get.

 

[flbchbm]

Geez, I get LESS than 25 per the car computer around town. Strat or dizzy tune. My baby screams to be held. To the floor. LoL

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[CarGuy]

I'll be brief. Just recorded best mpg ever with me driving, 39.1 mpg. Then, 26.9 out of nowhere. No changes in highway/city mix, driving style, etc.

Things that happened in order of note:
-oil change on 10/5
-cleared old dtcs from several months back
-record 38.2 and 39.1 mpgs, sequentially
-filled up with gas and it stumbled as if i had tried to over fill the tank but i stopped at the first click as always
-yesterday noticed i was at 115 miles on a 1/2 tank. Usually closer to 200.
-cleared oil light
-recorded 26.9mpg

Other notes:
-AFR hovers around 14.7 under constant throttle, around 40 when cruising/shifting (no throttle), drops to 12.4 under heavy throttle
-seemingly no loss of power, boost seems normal for a tune, ~23psi
-oil may be slightly overfilled (can't see the word "max")... But two of my best mpgs ever with it overfilled?
-disassembled and reassembled intake to verify no vacuum leaks
-no smell coming from the brakes
-accidentally pressed one of the cruise control buttons and RPM gauge dropped to 1k for a second...but engine RPM did not change.

What am I missing? Ask me questions. Just a bad tank of gas? Something else?

Are you going by the trip computer or calculated mileage?

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[scooter123]

Did you fill up just before this drop in mileage? Was it at a Sunoco? If so you just got defrauded by an Octane Cheater. Quite common at the Sunoco's on the New York thruway and getting 86 octane when you paid for 93 just plain sucks. Note, my experience with this crap dates back to 1989 and is still going on. It's why I'll only buy Sunoco gas if the tank is darned near empty and even then I'll just purchase what it takes to to get me to a better brand of gas. BTW, my Preference is Kroger's, BP, Amoco, and Marathon with Shell being sort of OK in a pinch.

BTW, last May I filled up at a Speedway because I didn't want to take the time to go to Kroger. Paid the price for that with fuel economy that was off by 5 MPG on the trip down to Ligonier, PA. Filled up at an Amoco down there and the trip back yielded 39 mpg with me thrashing my ST a bit because I wanted to get back home as quickly as possible. I'll also note that back roads in SE Ohio can be rather fun.

 

[Negi]

So i swapped to Map intake from drop in filter.After a few days of driving my cel came on for the Purge valve,car is a 19 with 17k miles stage 2 with dizzy.Cleared the code and my Oar dropped to -66 from -1.So babied it till it was almost empty,in that time the Oar creeped back to -90.I always fill up at the Sunoco in town.Got gas at a different station yesterday and the Oar is creeping back and its at -97.After or Before the Cel i have never had the typical symptoms of ruff idle or stalling.So i am assuming i got bad gas at the Sunoco.

 

[TURBO BLACK]

within the past month both of my cars have lost gas mileage and i use racetrack gas in florida.has to be something in the gas.

 

[358R]

Are you going by the trip computer or calculated mileage?

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Whatever the dash said. 395 miles on 10.1 gallons and now 272.2 miles on 10.1 gallons.

 

[akiraproject24]

-accidentally pressed one of the cruise control buttons and RPM gauge dropped to 1k for a second...but engine RPM did not change.


I do know that with some tunes pressing the cruise control buttons can change your map. When this happens it will hit the rpm numbers i.e. 1,2,3 which indicates the map you chose. It sounds like whatever cruise button you hit set the map to "1". Maybe you were previously running on another map and changed it.

 

[CarGuy]

Whatever the dash said. 395 miles on 10.1 gallons and now 272.2 miles on 10.1 gallons.

The on board trip computer is not very accurate. I've found over many fuel logs that the trip computer is off by 2-5 mpg per tank and if the battery gets reset as in pulling the negative cable to work on the car the trip computer is largely optimistic until you get a few tanks for it to "relearn" but is still off that 2-5 mpg over calculated fuel mileage. When I worked at Motor Trend we had a log book for each long term vehicle and I got used to logging my fuel and trip numbers and calculating that actual mpg. Now I use an app but continue to log every fuel tank, service, notes, upgrades, etc. into the digital log book. Old habits die hard I guess.

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[358R]

-accidentally pressed one of the cruise control buttons and RPM gauge dropped to 1k for a second...but engine RPM did not change.


I do know that with some tunes pressing the cruise control buttons can change your map. When this happens it will hit the rpm numbers i.e. 1,2,3 which indicates the map you chose. It sounds like whatever cruise button you hit set the map to "1". Maybe you were previously running on another map and changed it.

It's a Mountune. When checking "Show Current Map", the most recent one comes up. Though, i did notice it says "w/ 4 slots"...which I've never looked at before. To your point and me being relatively ignorant about Cobb APs (3rd owner of car), is it possible I switched to a different slot or something? I was driving when i hit whatever button. I feel like it would not/should not be able to switch maps on the fly while driving. Seems like it would be dangerous to the engine.

 

[CarGuy]

It's a Mountune. When checking "Show Current Map", the most recent one comes up. Though, i did notice it says "w/ 4 slots"...which I've never looked at before. To your point and me being relatively ignorant about Cobb APs (3rd owner of car), is it possible I switched to a different slot or something? I was driving when i hit whatever button. I feel like it would not/should not be able to switch maps on the fly while driving. Seems like it would be dangerous to the engine.

Google how to change Accessport maps and you'll see it take a few different maneuvers that normally wouldn't be used together. That way you can learn about how to use it too.

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[358R]

Google how to change Accessport maps and you'll see it take a few different maneuvers that normally wouldn't be used together. That way you can learn about how to use it too.

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Yep, just did. Seems that is a thing. Good info. Thanks. We'll see how this tank of gas goes.

 

[CSM]

You really can't trust the MPG read out. Calculate it manually.

Sounds like the slot change or clearing the codes may have caused the ECU to reset its MPG calculator logic. I always notice really bizarre MPG / Miles remaining readouts when I switch maps .

Nothing to worry about though. Carry on

 

[Intuit]

You really can't trust the MPG read out. Calculate it manually.
..............

What he said.

 

[iammacey]

Whatever the dash said. 395 miles on 10.1 gallons and now 272.2 miles on 10.1 gallons.

You're saying you filled up, drive the car 395 miles, filled up and it held 10.1 gallons? Then you drove 272.2 miles, filled up, and it held 10.1 gallons again?

The car holds around 12.2 gallons of gas. If you're filling up and looking at your estimated range, or distance until empty, that's not an accurate reflection of actual mpg. If you use the MPG view from the onboard computer that's also not exact.

Pushing a tune or clearing the ECU will show an estimated range of nearly 400 miles. And it takes time for the ECU to learn and adjust. Even resetting statistics when fueling up pulls some recent historical data to calculate range. I can drive on the track where I'm in the teens on MPG, and it takes a couple of tanks for the ECU to relearn and adjust projected miles until empty.

Fuel up. Drive the car. Fuel up. Divide miles by gallons. Get your actual MPG. There are apps for tracking this.

Fuel, temperature, time at idle, traffic, start/stop, and most importantly the driver all factor into fuel mileage.

I would guess nothing is wrong. Depending on when you pushed the tune, I'd guess it's more possible the ECU was cleared and it's taken two fuel cycles to adjust and learn.

 

[Sam4]

what is this Merlin in our Midst?? Math by physical activity? Next the sun's gonna be in the center of the Uni!

 

[358R]

You're saying you filled up, drive the car 395 miles, filled up and it held 10.1 gallons? Then you drove 272.2 miles, filled up, and it held 10.1 gallons again?

Correct.

The car holds around 12.2 gallons of gas. If you're filling up and looking at your estimated range, or distance until empty, that's not an accurate reflection of actual mpg. If you use the MPG view from the onboard computer that's also not exact.

Pushing a tune or clearing the ECU will show an estimated range of nearly 400 miles. And it takes time for the ECU to learn and adjust. Even resetting statistics when fueling up pulls some recent historical data to calculate range. I can drive on the track where I'm in the teens on MPG, and it takes a couple of tanks for the ECU to relearn and adjust projected miles until empty.

Fuel up. Drive the car. Fuel up. Divide miles by gallons. Get your actual MPG. There are apps for tracking this.

Fuel, temperature, time at idle, traffic, start/stop, and most importantly the driver all factor into fuel mileage.

I would guess nothing is wrong. Depending on when you pushed the tune, I'd guess it's more possible the ECU was cleared and it's taken two fuel cycles to adjust and learn.

Thanks. Yea, i did my own math. I would never trust anything estimated by the car. Seems as if it were a bad tank or similar to what you said about when i cleared the DTCs/pushed the tune. Seems fine now. I've had (2) 32.x MPGs and another 38.x MPGs.