2005-2014 Mustangs Discussions on the latest S197 model Mustangs from Ford.

Drop in MPG

Old 07-29-2009, 09:43 AM
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Default Drop in MPG

I have a 2006 GT 5-speed with 8600 miles.

I do mostly city driving and used to get about 10 - 11mpg (sometimes up to 12/13). Recently I started getting 7 - 8. Any possible mechanical causes I should check? I don't think i've changed my driving style or anything else I can think of. I am taking it to the dealer for other (trim) repair and will mention it but just want to have some idea if possible.

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Old 07-29-2009, 09:57 AM
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how are you on your maintenance? change the oil yet? clean out the air filter? things like that can cause the MPG to drop off.
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Old 07-29-2009, 10:21 AM
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WOW! 10 mpg! What are you running- 4.30:1 gears, and a superchager?! J/K

Assuming nothing is mechanically wrong- low tire pressures, dirty oil, ambient air temp. (small change as the EEC will adjust for this), dirty body (paint, glass, etc).

There are a number of things that could cause fuel mileage problems, but most of them would be apparent as you're driving the car. For example, if your IAC were malfunctioning (throwing the a/f ratio off), you would notice it idling strangely.

I would also check to see that you're not dragging anything. I realize that you're referring to city driving, but anything that is hanging under the car would get worse the faster you go.
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Old 07-29-2009, 11:09 AM
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It's also possible that you had been using normal gasoline all along, and then filled up with E10 gas at the latest fuel stop, which has less energy content per gallon than normal gas and thus reduces gas mileage.
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Old 07-29-2009, 11:12 AM
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yeah, less content per gallon, and the price goes up.
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Old 07-29-2009, 04:35 PM
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Where do you live? I live in Texas and I have noticed over the last couple of years that in the hotter parts of summer my MPG drops about 1-1.5 MPG. I assume its the hotter air thats getting sucked in.
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Old 07-29-2009, 04:42 PM
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How many tanks have you gone thru since you've noticed the drop and have you reset the onboard computer with each fillup? I wouldn't base a mileage drop on just 1 or 2 tanks.

As mentioned above, ethanol will impact mileage depending on the percentage. Also check your tire pressure. I assume you're not seeing a CEL since you didn't mention any.
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Old 07-29-2009, 07:40 PM
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you should probably change your fuel filter

I'll bet that thing is full of crud

after 3-4 yrs of essentially sitting, the gas has probably gotten some nasty stuff built up and that most likely is clogging the fuel filter.

sounds strange, but a clogged filter can eat up your MPG's

otherwise, how old is the oil/air filter? your tire PSI correct?
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Old 07-29-2009, 07:44 PM
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How are you calculating your fuel mileage?

Mixed driving is hard to get anything usable. Next time you go on a trip, start with your tank full. When finished, fill it up.

Letting your car idle makes it worse. Running the AC drains some too.

Hitting the reset on the dash for the mileage and then checking it isn't that great either. Don't throw parts at it until you confirm the mileage.
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Old 07-29-2009, 07:51 PM
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Just like others said, general maintenance on the car, just follow the manual but being in the city I'd go every 5,000 even 3,000 to be safe. Personally 7,500 is too much without doing checkup on your car.

But 10/11 MPG? That's pretty low man, not sure what to say.
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