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Old 07-28-2022, 02:46 AM   #1
trashboy
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cwp

Default absolutely horrid mpg after going stage 1 (pro tune)

is it normal to have absolutely terrible gas mileage after tuning (2020 sti)?

for context, when i was stock, i was getting around 20-24 (mix of city/highway) without caring about conserving gas.

i went stage1+ with a cobb sf intake and the ots tune, and again, without caring about my gas mileage would get about 20-24 (mix of city/highway). so it didn't really affect my mpg as far as i could tell.

yesterday, i got protuned and noticed my mpg was super low. so i did a test. i drove 20 miles (1 of which was city). never shifted past 3k rpm, never went past 70 on the freeway, and got 12 mpg. am i wrong in thinking this is insanely low, or is this just one of the things that comes with tuning the car.
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Old 07-28-2022, 03:16 AM   #2
D-Rodman
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That does not sound right.


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Old 07-28-2022, 09:00 AM   #3
whozatguy
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Did you add any new parts when you got the pro-tune, or was it just a custom tune for the intake and everything else remained the same?
Is the tuner someone who is experienced with Subarus? It sounds like something isn't right, a good tune shouldn't drop mpg like that.
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Old 07-28-2022, 09:17 AM   #4
undyjr
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That's the mileage I get on E85.
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Old 07-28-2022, 09:33 AM   #5
Max Capacity
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My car gained mpg with cobb AP and custom tune. Been that way since I installed Cobb AP V1 back in Dec 2004. Highway miles at 70mph or a little slower will get 26mpg +.
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Old 07-28-2022, 02:45 PM   #6
trashboy
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he tuner got back to me and said "the car calculates the mpg by using the maf calibration table amongst other things to come up w a number. compare with what it thinks you will get per tank and calculate your actual mpg when you fill your fuel tank at the station. also look at your af correction and make sure it isnt trying to add a bunch of fuel unnecessarily"

so i think it may be a misrepresentation of my actual numbers rhat iím seeing. i filled up my tank, am going to run it about half, and divide the amount of miles driven by the number of gallons on my next fill up to get my true mpg. i will say that it (the reported mpg) doesnít seem to be accurate. iíve driven a good amount since the tune and didnít think it was draining any faster than stock. will report back on my next fillup.


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Old 07-28-2022, 02:48 PM   #7
trashboy
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cwp

Default absolutely horrid mpg after going stage 1 (pro tune)

Quote:
Originally Posted by whozatguy View Post
Did you add any new parts when you got the pro-tune, or was it just a custom tune for the intake and everything else remained the same?

Is the tuner someone who is experienced with Subarus? It sounds like something isn't right, a good tune shouldn't drop mpg like that.


no i didnít add any additional parts. the only thing itís tuned for is the intake. the tuner is a reputable subaru tuner and has an 02 wagon if his own. he was showing me different cars heís tuned and when he was scrolling through his customer list, i saw lots and lots of subarus.


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Old 07-28-2022, 07:01 PM   #8
a5m
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I always calculate by hand. Possible the ECU calculation is just wrong.
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Old 07-28-2022, 07:46 PM   #9
K3rm1tth3fr0g
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Your ECU MPG calculation isn't going to be correct unless your tuner adjusted the scaling of the multiplier
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Old 07-28-2022, 07:48 PM   #10
djoye
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I think it takes an extra commitment by the tuner to get the electronics to accurately report fuel mileage after a tune. I'm guessing Cobb's shelf maps are all set because I used their map and don't remember the MPG estimate being wrong before I got a custom tune, after my custom tune it wasn't accurate (bigger injectors apparently affect it). I later went back for another tune and asked the tuner if he could tweak the fuel mileage display settings, it got better, but it's not perfect.

https://cobbtuning.atlassian.net/wik...lay+Correction

From the linked page:
Quote:
To adjust values based on manual fuel mileage calculations, apply the following formula: "(FMD Mileage Value / Actual Fuel Mileage Value) * Current Table Correction Value".
I'm guessing that if you calculate your mileage manually, then provide that number and the number displayed on your fuel mileage display at time of calculation, your tuner can then multiply that value by the correction value that's currently set and improve the accuracy.
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Old 07-29-2022, 12:53 AM   #11
K3rm1tth3fr0g
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Link flex tuned 02
WRX EJ205 20G-XTR JDM 6MT

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Quote:
Originally Posted by djoye View Post
I think it takes an extra commitment by the tuner to get the electronics to accurately report fuel mileage after a tune. I'm guessing Cobb's shelf maps are all set because I used their map and don't remember the MPG estimate being wrong before I got a custom tune, after my custom tune it wasn't accurate (bigger injectors apparently affect it). I later went back for another tune and asked the tuner if he could tweak the fuel mileage display settings, it got better, but it's not perfect.

https://cobbtuning.atlassian.net/wik...lay+Correction

From the linked page:


I'm guessing that if you calculate your mileage manually, then provide that number and the number displayed on your fuel mileage display at time of calculation, your tuner can then multiply that value by the correction value that's currently set and improve the accuracy.
This ^
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Old 07-29-2022, 06:25 AM   #12
trashboy
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as an update, talked to my tuner. he checked the map and noticed "some things changed with the fuel economy display that shouldnt have been changed". sent me a new map, i retested using the exact same methodology and got 27 mpg. so all is good.

thanks everyone
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Old 08-12-2022, 11:18 AM   #13
alexu27
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My car gets an average of 12.9 mpg with a protune as well. My tuner says its normal but I'm not sure if I trust him 100%
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