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Old 09-22-2006, 08:14 PM   #1
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Horrible Gas Milage!

I've got a 1998 VW Jetta GL with the 2.0 liter engine. And Im getting HORRIBLE gas milage. I've done a couple of performance modifications such as an intake, exhaust, and cam, but after I did those, I used to be able to get at least 24 MPG city. Now i get 22 MPG all city. Whats going on here?
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Old 09-22-2006, 08:27 PM   #2
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Performance modifications do not help fuel economy under normal conditions. They might even hurt fuel economy. The performance exaust is no benifit at low rpm.
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Old 09-22-2006, 08:52 PM   #3
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Quote:
Originally Posted by mexellent
Whats going on here?
There are many factors that can influence FE (fuel economy), and (without more info) we really can't tell which may be the cause in your case. However, the first thing I would suggest, it to check the FE basics. For example:

Tire pressure? Now that it's getting colder, tire pressure will naturally go down (unless you put more air in the tires to keep them at summer pressure). And lower tire pressure can cause your FE to tank! Which is why when it gets colder, you should check your (cold) tire pressure more often, and top off the air in the tires as needed.

What about oil changes, or other maintenance (including replacing air filters, cleaning your fuel injectors, etc)? If some needed maintenance is not up to date, FE can suffer.

Are you using different gas (different brand, or different octane rating) than you used in the past? Differences in gas frequently account for "unexplained" differences in FE.

What about driving conditions? Bad weather can lower FE.

Are you driving any differently? 2 mpg can easily be explained by simply having a little more of a "lead foot" than normal. And it can equally be explained by having to idle longer (caught in traffic, for example).

Are you using a different engine oil? I find that there really are FE differences (not to mention performance/power differences) between oils. I personally use a 5w30 full synthetic, and I get much better results than I did using a cheap dino oil. Oh yeah, don't forget to check the dipstick from time to time (in my wife's Civic, the FE really tanks when she lets the oil level get low)...

Are you overdue to replace your transmission fluid? Keeping clean "high quality" transmission fluid in your transmission, can make a little difference in FE (not to mention extending the life of the transmission).

Are you using more electricity (lights on more, for example) than you previously did? Many people assume that car electrical power is "free", when really the more power you get from the alternator the more drag the alternator puts on the engine (and therefore the more electricity you use in your car, the lower your FE will be, as that electricity takes some extra gas to produce). Which is why I have most of my car converted over to energy efficient LEDs (instead of the power sucking incandescent bulbs it came with), and also avoid using the (high energy using) headlights when they are not needed. But now that we are getting later in the year, the power sucking accessories (including the car lights) become more needed, and this can lower FE some.



And those are just factors "off the top of my head", I'm sure that list isn't even close to being complete. If you read the posts in this forum, you will see that many things can influence FE. So without knowing your car (or how you drive it), we can't really tell you what happened to lower your FE. But we can point you to things to look at that effect FE. And some of these tips would likely help your FE (possibly even giving you more MPG than you started with), even if you never find out why your current FE took a dip...
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Old 09-22-2006, 09:14 PM   #4
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First, welcome to GasSavers!

I'd review the FAQ section and take DracoFelis' advice.

It sounds like you made performance mods instead of economy mods (like a cold-air intake may be worse than a hot-air intake, for example).

We're here to answer specific questions if you have them.

The best way is to track mileage by using the GasLog here, and to get a feedback device like the ScanGauge to train your driving style.

Good luck...

RH77
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Old 09-22-2006, 09:39 PM   #5
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Could be your driving style has changed.
You may not even notice it. I know when i had exhaust/intake etc.... I became more used to shifting based on the sounds i was hearing.
Pay more attention to the rpms you shift at.
With that setup your engine will be passing more air, throughout the rpm range.
You may have to shift lower to compensate.
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Old 09-22-2006, 11:49 PM   #6
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thanks guys. Ill be sure and fix all those things. Ive got the stock cam. I might just stick that in later.
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Old 09-23-2006, 01:19 AM   #7
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Hi FormulaTwo

I agree with you ..

The difference was only 10% and that could easily be from a changed driving style - and higher shift RPMs etc.
And a car that starts at 24 isnt really that good anyway , a drop to 22 iant going to make much difference.
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Old 09-23-2006, 05:54 AM   #8
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Quote:
Originally Posted by theclencher
Careful...

Some mods do help performance and FE and some don't.
Yeah thats true.
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2008 EPA adjusted:


Distance traveled by bicycle in 2007= 1,830ish miles
Average commute speed=25mph (yes, that's in a car)
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Old 09-23-2006, 08:10 AM   #9
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Quote:
Originally Posted by theclencher
Careful...

Some mods do help performance and FE and some don't.
Back in my Uncle Ben days, I had a CAI and catback exhaust on my Integra. EPA was 25/31, I used to average 30 all city, driving like a typical impatient lead footed teenager. I still cant believe I once got 35 for the tank all city, but changing out my spark plugs, which were white tipped, gave me a good idea why I was getting the FE I was.
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Old 09-23-2006, 08:39 AM   #10
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Wha?

Quote:
Originally Posted by BEN_EJ8
Back in my Uncle Ben days, I had a CAI and catback exhaust on my Integra. EPA was 25/31, I used to average 30 all city, driving like a typical impatient lead footed teenager. I still cant believe I once got 35 for the tank all city, but changing out my spark plugs, which were white tipped, gave me a good idea why I was getting the FE I was.
Holy crap! What year/trim was the Integra? Also, was it a manual? I'm struggling to average 31 with all kinds of techniques and mods.
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