lousy gas mileage

[wtdash]

Our Co. has an '03 Forester w/AT and we're getting 26+MPG...on the winter gas.

[OB99W]

My tire supplier, a wholesaler only, advises that all tires should be run at their maximum rated pressure for best economy and to reduce tire wear. Underpressure results in weaker sidewalls and shoulder wear.

Partially true. The more air pressure, the greater the circumference and the less the rolling resistance; rolling resistance is something to reduce if better gas mileage is the goal. However, inflating to maximum pressure (when checked cold) may cause the center of the tread to wear faster than that at the shoulder area. Of course, if you tend to scrub tires during high-speed turns, that could even the wear out, but you'll go through tires fast.

 

Also, he advises that there should be no differential in front and rear pressures as the actual circumference of the tire depends upon the tire pressure, not the load upon the tire. This is a very important aspect for all owners of 4WD and AWD to consider.

Really ? Ever notice that a tire with no air in it looks perfectly round as long as the weight of a vehicle isn't on it? The load certainly does "flatten" the tire at the road-contact area, reducing the effective radius (distance from axle center to road). The greater the load, the more that happens.

 

I have requested an opinion directly from the manufacturer to confirm or dispute this position. In the interim, is anyone aquainted with a person 'in the know' from whom we could get an educated opinion?

It would be interesting to get an "official" response from a tire manufacturer, but some applied physics and observation go a long way .

 

I always went by the pressures listed on the door post, except for big mags which required full pressure to keep the center of the tread on the road. My tire buddy straightened me out a few years ago.

Vehicle manufacturers sometimes (it used to be more prevalent years ago) suggest a slightly lower-than-optimum tire pressure because it gives a "softer" ride. For those of us more concerned with handling and other factors, a couple of psi or so more than spec usually does the trick. Overdoing it can actually cause drivability to worsen. I once rented a car which had very obviously underinflated tires. When I pointed it out to the agency, they "fixed" the problem" for me. I began driving, and the car was basically uncontrollable; it wandered all over the road. A check of pressure later (cold) found they had inflated to 44 psi, the max rating for the particular tires . I adjusted the pressure to a bit over spec, and the handling was fine for the rest of my trip.

 

--OB99W

[cookie]

Tire pressure is a big subject. On my Forester I go by the door as I believe the manufacturer has done a bit of testing. (I hope). The weight of the car is not the same front to rear and inflation should compensate for that. Since I do everyday driving I use everyday drining numbers. For performance or hauling it would make sense to compensate for that with a bit more air.

 

I personally think the max on the tire is just that.. a maximum pressure not to exceed. Tire Rack's web site has tons of data on this and experts to ask.

[1 Lucky Texan]

If a car had 50/50 weight distribution, then the shape/size of the contact patch of identical tires with identical pressures would be the same. But our (and most) vehicles are front heavy and the front tires - for at least this reason, and maybe including braking, understeer,ect. - need MORE pressure than the rears to maintain a similar contact patch size.

 

 

Admittedly, there may be reasons that, given the dynamic nature of cars, even having the same size contact patch would be detrimental.

 

Personally, I approximate the 'door' specs plus around 10 percent. maintaining any difference front to rear. On the Outback when I knew I was preparing for a road trip with luggage and passengers, I ran the same pressures front to rear and about 20% higher than recommended.

 

I dunno

[OB99W]

Not that the tire pressure discussion hasn't been interesting, but I had a couple of other thoughts concerning gas mileage. The first is that San Diego tends to be on the warm side, even this time of year, so A/C use could be a factor. The other is speed; boxy (not "boxer" ) vehicles aren't exactly aerodynamic, and going significantly above "normal" highway speed will have more of an impact on them than those with "nice" shapes and ground-hugging. I don't know if either of these come into play for lytnin88.

 

--OB99W

[cookie]

over lunch I thought of a test that might be interesting. I am still doing the 15-50 Mobil 1 oil test but this would not interfere with that. By the way my oil sample is ready to mail to Blackstone but I don't feel like standing in line at the PO until after xmas.

I think I will run the next few tankfulls through with premium fuel to see if that makes any difference. a few folks have reported a difference for them so this should be interesting (and fairly cheap).

I am not sure why the best mileage I have seen reported by Foresters seems to come from up north. Do they have better fuel or driing conditions? Are the people saner?

[1 Lucky Texan]

over lunch I thought of a test that might be interesting. I am still doing the 15-50 Mobil 1 oil test but this would not interfere with that. By the way my oil sample is ready to mail to Blackstone but I don't feel like standing in line at the PO until after xmas.

I think I will run the next few tankfulls through with premium fuel to see if that makes any difference. a few folks have reported a difference for them so this should be interesting (and fairly cheap).

I am not sure why the best mileage I have seen reported by Foresters seems to come from up north. Do they have better fuel or driing conditions? Are the people saner?

 

Reset the ECU immediately after filling up. Other wise it could take several drive cycles for the ECU to advance the timing for the higher octane fuel.

 

fun idea

[cookie]

I was going to run a couple tankfuls though to get more premium in there to have less of a mix. I'll pop the battery cable off at the same time and leave it overnight.

By the way so far I think the Mobil 1 15-50 has made very little change in my fuel mileage. It may have improved it by less than a half MPG at best over the long run. Remember though this oil is a bit heavier than my former Dino oil and the only real result is less piston slap.

[otis]

This is such a tough issue to get a handle on. I've read all the posts about O2 sensors, tire pressure, winter gas, on and on and on. I've done some of my own unscientific testing comparing octane (reg vs mid), tire pressure (32 vs 36), and highway speed (65 vs 75) and the only thing that made a statistically significant change in mpg was the highway speed for my 00 forester auto (23-27ish for 75 vs 65 mph)

 

Now I've read through quite a few posts on this topic here. ultimately I would like to trade cars for a week with someone who claims they get 30 mpg to see if it comes down to driving style. I think if you normally get in the low 20's for mileage, then doing all the suggestions listed here, may only improve your mileage slightly (maybe 1-2 mpg) but not the 5-7 mpg claims that some people have UNLESS you change your driving style. [this is my opinion only- I'm NOT one of those "people in the know." I'm just basing this on my own observations so take what I say with a grain of salt.]

 

it just seems like there are MANY more posts where people report 20-25 mpg vs 25+ mpg. so I'm thinking the 20-25 is the "norm" whereas people who are achieving higher mpg must be easier on the gas pedal. YMMV:grin:

[OB99W]

I am not sure why the best mileage I have seen reported by Foresters seems to come from up north.

That's an interesting observation. Assuming we equate "up north" to "colder", there are a lot of factors to consider that could cause fuel usage to go either way...

 

1) The colder the air, the denser it is; affects A/F ratio, and therefore power and mileage. Also, it provides more resistance to driving through it -- go fast enough, and the result is noticeable.

 

2) If the driving conditions are bad (snow, whatever), drivers slow down, there's wheelspin, etc. If they slow down from otherwise high speeds, mileage improves. If they're slipping and sliding, mileage goes down.

 

3) During engine warm-up, which of course takes longer the colder it is, fuel consumption is usually up due to inefficiencies. Lubricants can get very sluggish in the cold, also adding to fuel usage.

 

4) Gas expands with temperature. You literally get a bit more from a cold storage tank (when the samples are equalized to the same temp). So gas bought cold improves apparent mileage.

 

...just some thoughts. I'm sure I left things out.

 

--OB99W

[OB99W]

it just seems like there are MANY more posts where people report 20-25 mpg vs 25+ mpg. so I'm thinking the 20-25 is the "norm" whereas people who are achieving higher mpg must be easier on the gas pedal. YMMV:grin:

My '99 OBW only gets about 20 (19 in winter) mpg with my usual driving, and I don't "get on it" very often. However, let me provide some details. I don't do a lot of highway driving. The region is somewhat hilly. When I do get on the highway for a prolonged time, the mileage has been about 26 mpg. Overall, I think the mileage is a tiny bit lower than I'd expect/like for mixed driving, but not "terrible" based on the EPA 21/26 rating.

 

--OB99W

[cookie]

Sorry when I said up north I meant Oregon and Washington. The colder air thereories could apply or maybe they get better gas.

I do like Otis's idea that it would be very interesting to switch cars with two folks that have identical equipment. I am sure driving style has a lot to do with it.

The best mileage I ever got from my phone booth, oops Forester, was driving through Big Sur. You would expect lousey mileage but I got my best tank ever at nearly 28. You just can't go fast in that area....

Best mileage I recall was from a woman up north who could hit 30 with one of these. I always though she might be a very smooth driver and live in a good area.

I supect the Legacies might do better than my car because although heavier, they look pretty slippery.

I think most of the 2.2 folks report a bit better mileage with thier lighter cars and smaller engines.

[friendly_jacek]

So did you do any mods/improvements that actually gave you an improvement in gas mileage. I am looking for a way to get better MPG and am looking for advice.

 

Nope, nothing helped. I should also add that I replaced O2 sensor and fuel pressure regulator to no avail. The only good news is that if I decided to replace the legacy with anything else capable of pulling the boat I have, the MPG would go south.

[cookie]

I was just wondering if you were close enough to San Mateo where we could switch cars for a week.

[bfg9k]

Nope, nothing helped. I should also add that I replaced O2 sensor and fuel pressure regulator to no avail. The only good news is that if I decided to replace the legacy with anything else capable of pulling the boat I have, the MPG would go south.

 

Did you replace both O2 sensors or just one?

[nipper]

and which o2 sensor

 

 

nipper

[nipper]

HA i got you all beat ..... i may have set a record. Just filed up the car, and i got 10.5 mpg.

 

 

 

Of course thats over a month, been driving one trip of 5 miles, and another 0f 2 miles every ther day, and using the remote starter a lot. Normally around town i get 20 mpg. It just goes to show a lot goes into gas milage, not just the car design.

 

 

nipper

[OB99W]

..... i may have set a record. Just filed up the car, and i got 10.5 mpg. [...] Of course thats over a month, been driving one trip of 5 miles, and another 0f 2 miles every ther day, and using the remote starter a lot.

The short trips don't help, but idling (getting 0 mpg) a lot certainly doesn't improve the average .

 

--OB99W

[cookie]

I guess we have to add non driing habits too. Happy holidays folks.

[nipper]

i meant to say it took me a month to burn through that 13.5 gallons (thats when the light came on). So im not really worried, but yes ALL habits have to be considered.

 

 

nipper

[friendly_jacek]

Did you replace both O2 sensors or just one?

 

Only the front A/F ratio sensor, even though the OBD2 software I use indicated that the sensor worked fine.

I didn't touch the rear one since again, no indication of malfunction and the rear one just makes sure the cat is working and doesn't change fueling, as far as I understand.

 

You might find it interesting that I replaced fuel pressure regulator because the fuel pressure was ~10PSI above norm. The kick is that with a brand new one is it still several PSI above specs (tested with and without vaccum line).

 

I'm not sure how to experiment with fuel pressure short of getting variable pressure regulator.

[bfg9k]

Only the front A/F ratio sensor, even though the OBD2 software I use indicated that the sensor worked fine.

I didn't touch the rear one since again, no indication of malfunction and the rear one just makes sure the cat is working and doesn't change fueling, as far as I understand..

 

I had the same experience with the front O2 sensor. I'm considering doing the rear with a new OEM sensor.

 

I wonder if these cars are maybe prone to a fuel leak somewhere. I've never seen one in the engine compartment. I get a really strong gas smell on cold mornings, although I don't know if that's any clue.

[Lesbaru]

Front O2 sensor on my gf's '02 Forester.

[Gnuman]

I had the same experience with the front O2 sensor. I'm considering doing the rear with a new OEM sensor.

 

I wonder if these cars are maybe prone to a fuel leak somewhere. I've never seen one in the engine compartment. I get a really strong gas smell on cold mornings, although I don't know if that's any clue.

Check all the connections on your fuel rails. Also do a search for "Fuel smell"

Info regarding this has already been gone over.

[friendly_jacek]

I noticed that unburned gas smell in my legacy too on first starts in AM. It is coming from exhaust and not from fuel system. That is why I believe the car runs too rich. Yet, everything checks OK. I belive that the fueling in 2000-2003 subarus is out of whack. Whether the mixture is set too rich, the fuel pressure too high, or A/F ratio sensors lie I have no idea. Notice that the MPG in newer Subarus is better IIRC.