Fuel efficiency mystery...any Subaru gurus out there?

[midnight armadillo]

I like my 98 Subaru L wagon well enough. It was the best car we could afford for my wife & baby last year, and now with my new job & a 2001 Saab 9-5 wagon for them I have inherited the Subaru as a commuting car. It was stellar in this morning's ice and snow with its Nokian WR tires.

It's rated for 23 city, 30 highway with its somewhat wimpy 2.2L 4-cyl engine. My commute is 90% highway (doing 60-70 most days), 10% city, and on the weekend I drive the Saab.

I get 21 mpg, below even the city rating, with the Subaru!! I have added new tires (the old ones sucked), which gave me a 1.5 mpg increase. I switched the car to synthetic ATF, synthetic gear oil in the differentials, and synthetic 5w30 engine oil. The car runs and shifts smoother, but no mpg change. New spark plugs, ignition wires, K&N air filter, and OEM fuel filter 500 miles ago - no mpg change. The car has 106k miles.

So car gurus, what could be wrong with my Subaru to get such sucky mileage?? I suspected it had a sticky left front caliper last summer but never checked it out. Could that cause such a drastic falloff? That brake is getting squealy again so I think it's time to take a look. Could there be a transmission problem? It runs at 2750 rpm at 70 mph.

[mr_hat]

1. We've always been a bit disappointed

with our Forester's mileage. The rather upright stance may hurt its aero.

The boxer 4 motor, too, has many virtues; fuel stinginess isn't one.

[One_Life_To_Give]

2. Compression Test

There are many possible reasons. A dragging brake should be noticable in your stearing. Unless both sides are dragging evenly. Have you checked motor compression? What did the plugs look like when you replaced them? Have you run a stall test on your Torque Converter?

[midnight armadillo]

6. The old plugs were crappy

I think they may have been the original ones on the car! I have some repair records from the prev. owner but they mostly cover oil changes and brake work.

How do you do a stall test on the torque converter?

[Deja Q]

3. My 99 Imprezza Outback does typically 21-23.5, but once in a while,

18.5 isn't out of the question.

[catmandu57]

4. More than likely it's your driving

70 mph is going to drag it down, slowdown if you can. My poor old glc wagon is getting 21 mpg sometimes 24 and I feel lucky to get that, but I keep it at 60-65 and let people get around me.

[midnight armadillo]

5. I just go with the traffic

70 mph is just the traffic flow on the highway. I don't drive aggressively or do a lot of lane changing, and about 70% of the time I manage to use the cruise control to maintain speed once I'm on the highway.

[punpirate]

7. A sticking caliper can...

... cause some loss of mileage, but I suspect you'd notice other problems, too, if it were sticking enough to cause a drop in mileage.

The first thing you should do is check the computer(s) for codes (can't remember if the `98 has integral transmission/engine computer).

The other thing to remember is that the EPA mileage figures are based on a computerized route done on a chassis dyno, with a fudge factor thrown in for environmental variables such as wind drag. It's rather rare for any car to meet or exceed the EPA mileage figures--even when new--and especially so for an all-wheel drive car.

The big differences people find with their mileage vs. EPA figures is the way they drive. What may seem a reasonable and prudent driving style to you may be far less fuel-efficient than the EPA cycle. The EPA figures are a guide for comparison purposes only. Winter driving also considerably cuts down on mileage--especially if your commute is done in hours of darkness where the lights are on most of the time.

The other issue with EPA figures is that the manufacturers have to live up to something like five years of all systems working properly for emissions purposes, with normal maintenance. If yours hasn't been maintained during the time you did not own it, then it's hard to say if is still holding to intended performance standards. A partially open engine thermostat can slow warm-up enough to cost you several mpg and accelerate engine bore wear and increase carbon deposits.

One of the things you did not mention is doing a compression check. Do that, too. An engine with all cylinders down 30-40 psi on compression will idle reasonably smoothly, but not be nearly as efficient as it should be.

As for rpm vs. speed, I think your particular year of car has lockup in most forward gears, not just top, so it would depend when lockup occurs in top gear and if lockup is happening properly.

Cheers.