New wheels/subs = poor MPG? No way.. - Pontiac G8 Forum: G8 Forums - G8Board.com
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post #1 of 14 (permalink) Old 01-14-2013, 10:39 PM Thread Starter
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New wheels/subs = poor MPG? No way..

Hey all, so I got my GT back in early summer and couldnt complain at all about it. Since then she got a few new things:

- Magnaflow cat back (came with car)
- Spectre CAI
- DBS rotors/hawk pads
- SLP firehawks (upgrade from stock 18's)
- Kumho Ecsta 4x 245/40/19
Plus some other cosmetic stuff. Still stock tune, headers, engine components etc. Also to point out I run 93 octane all the time.

I've always gotten about ~17 city ~24+ highway. Always thought that was exceptional since I dont have the lightest foot. But here is where it gets strange.

I installed 4 kicker subwoofers in the car and obviously disconnected the negative battery terminal. At this time I also put on the new wheels/tires at this time as well.

Got everything all set, connected battery. ECU obviously was turned off and I reset all the TPMS senors and filled accordingly. For awhile despite the half of a tank I had in the car the Distance read 68mi to go for a day or so. After some driving it corrected itself but is giving a rather low estimate. Likewise MPG has taken a dip to sometimes under 10mpg city and upper teens on highway. I took the car on a 250 mi trip (should be long enough to relearn again) and got the following averages according to speed:

21.4mph - 11.2mpg
68.2mph - 17.4mpg
72.1mph - 16.5mpg
89.mph - 29.7mpg (this was the really odd one)

Im slightly perplexed here. I understand winter fuel will cause a decrease but did I really just coincidentally get the 1st tank of winter fuel all season as this happened? Should I pull the negative cable again and let it reset? Tire pressure I check consistently and everything checks out perfects (car reads it accurately as well). Engineering mode says AFM is on as normal and I double checked the air filter to make sure all was fine. Maybe I'm just paranoid...any help would be much appreciated. Thanks.
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post #2 of 14 (permalink) Old 01-14-2013, 11:17 PM
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You say you have headers and no tune? That is your issue. Fuel trims reset and you are running super rich. Or do you mean stock manifolds?

You have to fill the tank note your mileage and drive for a while. Fill back up and calculate your average mileage.

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post #3 of 14 (permalink) Old 01-14-2013, 11:44 PM Thread Starter
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Quote:
Originally Posted by Supercharged View Post
You say you have headers and no tune? That is your issue. Fuel trims reset and you are running super rich. Or do you mean stock manifolds?

You have to fill the tank note your mileage and drive for a while. Fill back up and calculate your average mileage.

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I ment that as stock tune, stock headers, etc lol

Its been about 2 weeks, 2 tanks since, same results
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post #4 of 14 (permalink) Old 01-15-2013, 12:32 AM
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What was the OD of the stock tire and the OD of the new tire? A change in tire diameter with out a correction to the ecu could cause a miscalculation of the mpg.

If that's not it then drive faster? I got better mpg with mine when I drove faster too. I think with the taller gears the slightly higher rpm's just puts the motor more in its power band. Still that's ****ty mpg, maybe you did get a bad tank of gas.

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post #5 of 14 (permalink) Old 01-15-2013, 01:32 AM
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Not to say this truly is the issue ,its not but those subs are gonna weigh you down. What size are the four subs you put in ? Between the subs ,the box and well the amp or amps are kinda minor .plus if you are like some cars with an extra battery, all that is gonna add up .throw a couple friends in the car with all that and I'd think that could kill a couple mpg's. I'm currently getting around 19 highway and I am kinda heavy footed. My car is all stock for now . Plus I do use remote start and that'll lower it .
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post #6 of 14 (permalink) Old 01-15-2013, 07:34 AM
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I think the tire compounds can have an effect also. I remember reading posts about... I dunno, something I was researching and and the poster mentioned that while the wheel/tire combo he picked looked good and handled okay but the "rolling resistance" made his MPG drop.

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post #7 of 14 (permalink) Old 01-15-2013, 10:41 AM Thread Starter
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Quote:
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What was the OD of the stock tire and the OD of the new tire? A change in tire diameter with out a correction to the ecu could cause a miscalculation of the mpg.

If that's not it then drive faster? I got better mpg with mine when I drove faster too. I think with the taller gears the slightly higher rpm's just puts the motor more in its power band. Still that's ****ty mpg, maybe you did get a bad tank of gas.
From the math I did the difference in tire size made a 0.1mph difference reading so I cant see it effecting MPG that much.

Usually its only me in the car, but ive had her loaded up with friends/family and still never saw MPG like this.

Try seafoam and clean things out? Reset ECU again next full tank maybe?
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post #8 of 14 (permalink) Old 01-15-2013, 11:08 AM
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Has it been cold where you live? AFM does not activate if the temp is too cold (below 40 maybe). Also, if you let your car idle for long periods of time that will lower your average as well.

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post #9 of 14 (permalink) Old 01-15-2013, 11:45 AM Thread Starter
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Originally Posted by OrngG8 View Post
Has it been cold where you live? AFM does not activate if the temp is too cold (below 40 maybe). Also, if you let your car idle for long periods of time that will lower your average as well.
Well that could be an explanation, it's been about 30 here or colder.
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post #10 of 14 (permalink) Old 01-15-2013, 10:32 PM
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What do the new wheels weigh compared to the old ones?
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post #11 of 14 (permalink) Old 01-16-2013, 09:21 AM
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Switch the wheels back and remove the subs...Does the mileage return? Seems simple enough solution.

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post #12 of 14 (permalink) Old 01-16-2013, 09:22 AM
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Also..do you know how to check your mileage "the old fashion way" with a calculator? Try that!

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post #13 of 14 (permalink) Old 01-16-2013, 09:24 AM
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When you disconnected the battery, you reset the computer. Maybe your mileage was high because of the previous owner not having a heavy foot and because it is an overall average calculation, your driving habits wouldn't affect the total average much. Now that it is reset, it is more true to your driving. Just a thought.

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post #14 of 14 (permalink) Old 01-16-2013, 05:12 PM Thread Starter
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Well today was 28 outside, started her up in engineering mode and AFM was fully active. Millage about the same.

Wheel weight: they seemed lighter by hand but it could never be that large of a difference.

I think my next step is going to be to run the tank close to E. Fill back up with fresh 93 and take it on that same 250mi drive. Might seafoam it and reset the ECU again this weekend before all that goes down.

Edit: What do you guys think about changing the spark plugs? The car just hit 60k and I assume their factory.
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