Sudden drop in MPG

[Enders]

A quick background of my car:

I bought a used 2007 WRX (not STI) with 36k miles on it in April, but didn't drive it until mid May due to school(left at home). I had the clutch go out on me towards the end of June. I had only put ~500 miles on it so with the extended warranty we had it replaced for free. As of now the clutch has about 2.5k miles, other than that everything else is original and stock(that I know of) with no mods on it. The mileage is a little over 39k.

My problem is now that I used to get about 18-20 MPG city. Now, however I'm getting around 14. I don't know the exact math, but I've driven 140 miles in almost 3/4 a tank, so I'd say I'm getting around 13-15 MPG city. Anyone know a reason for a sudden drop in gas mileage? I do need my oil changed soon, but would that cause a huge drop in gas mileage.

[teflon_jones]

You shouldn't see a sudden drop in gas mileage from oil. It could get slightly worse with old oil versus new oil, but it's probably measured in tenths of an MPG.

Are you sure you don't have a boost leak somewhere? Clogged air filter?

[mudferret]

Do a couple of fill ups, recording mileage and fuel added, it's hard to guess with the WRX gauge since what the needle says isn't always proportional to what's in the tank.

Where do you live? Could also be the transition to winter gas.

[SeattleJeremy]

I feel like I've had this conversation before.

The Gas gauge is not accurate enough to measure your gas mileage.

Fill up the gas tank. Get a printed receipt, write Odometer on the receipt. Reset the trip OD (not necessary, but I find it handy). Drive it until the light comes on. Repeat.

Miles driven / Gallons used = MPG

The oil in your car needs to be changed regularly, just like any other car. When was the last time it was done?

[nunez07c]

Same here, I really don't know why my MPG is bad. I drive from my home to Campus twice a week (home to school and back = 44 mi.) And I just put in a new vacuum(turbo) hose.

[Enders]

I always reset the gauge when I fill up(or one of them, I usually alternate). I still have past reference to the last fill up. I need to fill up today as I'm leaving for home for Thanksgiving break, so I'll let you know the exact MPG.

I live in Illinois, but this weather has been so strange lately I can't base it off that (it was in the 70s last week). My last oil change was JUST under 3k miles ago, so when I get home I going to get it changed really quickly. I still have the car under 2 year 60k mile warranty so I THINK any repairs I have will be paid for. In the past the gas gauge has been accurate for the most part, but this is such a sudden and large drop, and I have no mods which leads me to believe there's a mechanical problem.

[mudferret]

Look for repeatability, I wouldn't put much stock in one bad instance of mpg unless it was EXTREMELY bad.

Temps wouldn't lead me to believe that's the cause, but I'm assuming Illinois would be switching to winter gas around this time of year. I typically see horrid mileage for a tank or two until the ecu adjusts, and then slightly lower than usual mileage throughout the rest of the winter.

[anxiouz]

^^ That's a good point.

I would run a few full tanks of 91+ octane to see if things improve. A bad batch of gas can impact MPGs. Since the car is stock and you have an extended warranty, if you have the time it wouldn't hurt to drop the car off at a dealer and let them diagnose.

And I agree to use the write-odometer-mileage-on-receipt method and not just go by what the car is telling you. But it sounds like you'll still have the same result you're seeing.

It sounds like you don't drive the car every day so it may be hard to answer, but does the car drive any differently than you remember? Is the same power there when you floor it? Is the idle smooth?