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Modifications vs Gas Milage.

4K views 43 replies 25 participants last post by  Heavy-Fuel 
#1 ·
To all the people who have modified their G8, did you see any improvement in fuel economy? I was wondering if you bolt on x-amount in horsepower upgrades, would the car get better gas milage?
 
#3 ·
An intake and exhaust can actually get you better gas mileage. I see and increase of about 2 mpg on the highway with my mods...but it really still all depends on how heavy your right foot is.
 
#15 ·
2nd that, conservative driving is hard though:whine:
 
#4 ·
I'm only getting 16 around the city but that's alot of idling at stoplights (avg mph is 27mph). Highway I can get 27 or 27 at 70 mph.

Intake & tune are going to bump you up a couple mpg, anything after that will likely hurt. Keep in mind, running an aftermarket tune will probably increase the octane reqmt to 91+ so any increase in mileage is cancelled by more expensive gas.
 
#10 ·
... an aftermarket tune will probably increase the octane reqmt to 91+ so any increase in mileage is cancelled by more expensive gas.


unfortunately, the GXP has the premium fuel requirement from the factory ...

i find it amusing how GT guys casually make these kinds of comments in this GXP section ...
 
#5 ·
Intake, Tune and Exhaust modifications are all designed to increase efficiency and therefore they will increase gas mileage. Any modding beyond these basics usually results in a decrease in gas mileage. My car has seen as high as 29mpg with d.o.d. turned off over the course of a long trip but I usually get closer to 15 or 16 mpg because driving for mpg's isn't nearly as fun as driving for rpm's
 
#6 ·
Without a doubt, most performance improvements also come with MPG increases.

However, if your USING this new power, you'll likely see less MPG.

For the most part though.....CAI, headers and/or cat back, high flow cats, etc should show solid increases in MPG.

The AFM/DoD removal/tune out, can hurt MPG's.

But I noticed on a 2500mi round trip road trip at the end of February, that mileage really decreases after 65mph. I-95 has a 70mph speed limit just south of DC, all the way to Florida. This means people are rolling at 75-80. I set the cruise at 79 both up and back and got 24.7 average for the whole trip. We did minimal driving while down there. Average speed was 67mph when we got home!

My only mod is a Cortex 91 tune. Everything else stock.
 
#7 ·
Modifications make me want to tromp it at a higher percentage of the time.

thus, my gas mileage suffers considerably.
 
#9 ·
Added my CAI, headers, and tune all at once. My MPG has gone from 14.9-15 mpg to 15.5-15.6 mpg. Not a huge increase, but not a decrease either. 99% of my driving is in town.
 
#11 · (Edited)
GXP stock I was getting about 26-28 highway under ideal conditions, added headers/high flow cats/ tune and was able to squeeze 30-31 ideal highway driving conditions. Added a supercharger to the mix and Im lucky to see 21 MPG ideal conditions.

So opening up the breathing and tuning absolutely impacted the mileage positively. Once you add in forced induction with higher flow injectors, mileage kind of goes out the window.
 
#12 ·
Back when I had my 04 Chevy Cavi after I modded it with an INJEN intake, headar, hi-flow cat and 2.5 inch exhaust then ported the TB and intake and header it would get 42mpg on the highway with the cruise set at 65mph. The car had the 2.2 ecotec and was a 5spd LS.

Mods DO improve gas mileage till you start useing them. ;)
 
#16 ·
On a V6 all I did was add a CAI and no I really didn't notice any earth shattering MPG improvement. But I did clean my MAF sensor and Throttle Body today, it really needed it after 37K miles. If you have never done it, I highly recommend it. Both needed to be cleaned.
 
#18 ·
Then please explain that to my mpg calculations after putting one on...
 
#19 ·
Changing the intake (specifically a high flow filter and smooth intake ducting) would definitely improve mileage.

If something you add gives you horsepower on a dyno, then it has the ability to offer increased efficiency.
 
#20 ·
I have only really checked my highway mileage accurately, to tell snooty tree-huggers that my car can be Sick-Fast and Fuel Efficient as needed. These are my numbers totally stock. I have not had a real opportunity to test after my mods.

HWY 70-73mph on cruise for 2 hours: 24.8 (Verified a couple of times)
HWY 65mph on cruise for 45 minutes: 27.7 (One time test)

I cannot imagine the mods make it worse, only my right foot can do this :)
 
#24 ·
My car hovered around 22-23 MPG's on the freeway stock. With my current mods, I consistently see 25 MPG's (and sometimes a little better) between 65 and 70 MPH. There is no doubt that intake and exhaust mods improve efficiency. I find that fact ironic since the tree hugging idiots at the CARB typically frown upon mods, unless the manufacturers pay extra for their stupid E.O. number.
 
#26 ·
I haven't seen an improvement yet....so I'm going to keep modding until I do :pir_flag:
 
#28 ·
I recently drove my stock gt from Warner Robins to San Antonio...first tank was 24.5 driving at night through some mildly hilly terrain, w/ no A/C, @ 80-90 no cruise. Second tank was 25.5 on I-10 with the cruise set at 75 no A/C until last two hours. DIC was reset each fill up.
 
#33 ·
Science isn't always right...maybe you could ask some others that had the same results as me, because there are more of them in this thread.
 
#34 ·
in my 09 gt i got kooks long tube headers hi flow cats x pipe corsa exhaust CAI and tune and i got 27 mpg on i-95 yest
 
#37 ·
FWIW and completely unscientific, but like jhilliard14, after I put the Roto Fab on, my long time, rock steady, 18.2 mpg during my normal commute went to 18.5 mpg.

Of course, now that I have made other 'changes' I am down to 17.6 mpg on that same drive.
 
#39 ·
I still don't understand why you are aiming your claims solely at me when others have said the same thing.
 
#40 ·
Initial reason: Most people let it drop when I show the study. I have a little bit of ego pushing me to convincing you.

Current reason: You've carefully monitored your car and it's performance. Your attention to such details would be really helpful in finding out what all happened. I think everyone would like to know, and if possible to recreate the circumstances.

Please don't take it personally; I don't think anyone else could help as much as you could. If you don't mind, I'd like to know what, when, and how you've modded your car.
 
#41 ·
Reading through all the post, it looks like I cannot go wrong with a few modifications, since I only get about 14 mpg in town and 22-24 mpg on the highway.

I was going to do the intake and cat back exhaust, but you ya’ll think I should replace the cats and add headers? If I replace the cats and add headers, will I need to get a tuner? I am a little confused about tuners. I have been reading through the technical section about tuners and most I have seen are only for the 6.0 engines.

Any help would be greatly appreciated and thanks for all the input.
 
#43 ·
Reading through all the post, it looks like I cannot go wrong with a few modifications, since I only get about 14 mpg in town and 22-24 mpg on the highway.

I was going to do the intake and cat back exhaust, but you ya’ll think I should replace the cats and add headers? If I replace the cats and add headers, will I need to get a tuner? I am a little confused about tuners. I have been reading through the technical section about tuners and most I have seen are only for the 6.0 engines.

Any help would be greatly appreciated and thanks for all the input.
Do the headers and a cat-back, then call Livernois. Tell them what you have, and they can send you a tune for your set-up.
 
#42 ·
I'm assuming you have a GXP.

We don't know any other specifics....A6 or M6? (Hint: Your signature is a great place for this!)

A good cat-back exhaust will remove your second set of cats, replace your center muffler, replace your resonators, and of course replace your rear mufflers. It will give your more horsepower, and by doing so, should improve fuel economy slightly. You traditionally do not need a "TUNE" for this.

If you do anything to improve exhaust flow past your rear Oxygen sensors you will need a "TUNE" to disable their info from throwing a code. This would include almost all headers (shorty's, mids, and longs) as well as the simple addition of high flow cats to your stock exhaust manifolds.

A "TUNE" is not a hand held programmer (SuperChips Cortex) they are two separate things. A SC-Cortex does not work with the GXP.

Your mileage sounds about spot on.

Hope this helps.
 
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