Last post on Aug 18, 2008 at 6:25 PM
You are in the Subaru Legacy & Outback
What is this discussion about?
Subaru Legacy, Subaru Outback, Fuel Efficiency (MPG), Sedan, Wagon
#28 of 49 Re: Subaru Outback 2008 MPG [julecat]
Jun 18, 2008 (5:00 pm)
A lot will depend on your driving style. Certain weeks at a time I drive more... aggressively... both on the highway (NJ GSP/TP) and in the city (Newark, NJ). If I am in a rush and I'm tach'ing at 4.5k-5k RPMs when I shift, I will get noticably lower economy compared to, say, being gentle and shifting between 2.5k-3k RPMs.
Another big thing too is inertia. Objects in motion tend to stay in motion. Rather than accelerating into a red light, a prolonged brake which slows you from 50mph to say 20mph, then have it turn green and reaccelerating is better than 50mph to 0, waiting 10 seconds, then reaccelerating back again.
Another thing - highway speeds deteriorate due to parasitic drag (roofracks, too) which also includes the skin of the automobile (wash your car more often, wax it.) etc. This parasitic drag increases with speed exponentially - just like on an airplane (I'm a pilot, had to study this crap) - and you'll really notice beyond 55mph you start getting around a 10% loss in fuel every 10 mph you go - maybe more, if you have accessories like A/C running.
I noticed on a trip from NJ to FL, I had amazing mileage through NJ, MD, and DE where the speed limits are lower - 55 mph in some parts - compared to VA, SC, NC where the speed limits are 70. It made the difference between 450 miles per tank (11.9 gal tank on my Civic) and 350.
Keep in mind that your sticker rating on the side of your car is based on a highway speed test at 55mph for the highway rating. If you have a lead foot, like my wife, then you really may want to consciously try slowing it down - just 5 mph or 10 mph for a week - and see if there's a difference.
Typically, on my average commute of 10 miles, with an average speed limit of 55, it takes me about 20 minutes. Going 5mph faster would only get me there about a minute or two faster, but I don't feel like getting pulled over by the cops that wait in the bushes (they're in the same spot every day, haha!)
Ethanol does NOT have as much bang for the buck compared to gasoline, and by that, I mean energy measured in joules per given volume of liquid. In addition, lower octane fuel doesn't give as much energy per joule compared to plus or premium.
I'm not recommending you go and buy plus or premium unless you do some quick math to find if the extra mileage you're getting (say 50 more miles to a tank with premium, but you're spending an extra 10 bucks at the pump), is worth it.
So, if it's 25 cents more per gallon to run premium, and you get 15 gallons, thats 3.75 more for premium gas.
That's a little under what a gallon of regular goes for around these parts in NJ, so you'd have to figure you'd need to get at least 25ish miles per tank in order to justify paying the initial premium.
This takes a little investigation on your part, but you may want to look into it. I found by running Plus Test (91) in my tank, it costs me an extra 1.00 or so at the pump when I fill up, but I end up walking away with another 50 miles. That makes up for the extra gallon and a half of regular I'd need for my Civic - so I've started filling it with Plus.
I knew math would come in handy some day.
#29 of 49 Re: Subaru Outback 2008 MPG [pilot1226]
Jun 21, 2008 (2:25 am)
I just thought I'd also comment I don't condone everyone running out and buying premium fuel - the octane rating is simply the fuel's resistances to detonation (premature ignition). Higher-end cars that call for premium gasoline have a higher compression ratio in the combustion chamber (the specific ratio is typically found under the performance specs of the car), so the fuel needs to resist spontaneously combusting when it enters the chamber.
I would keep in mind too that you could - keyword COULD - run into fuel injector issues if you continuously run 93+ octane in a car that says "Unleaded 87 or better". I usually run plus test when I have long road trips, and a tank of premium every few months.
If you want more information on why not to run premium constantly in a car built for 87 octane, Google it. There's a lot of returns.
#30 of 49 Re: 2007 2.5 base automatic, 34 mpg. [peralta]
Jun 24, 2008 (6:54 am)
While we're all talking about MPG's, I figured I have a deep thought for us to discuss:
Gasoline is sold by volume (gallons in the USA). Temperature is directly proportional to density (If you have thing "x" at two temperatures, one higher and one lower, the lower temperature one is more dense.)
My thought is, if you purchase gasoline in the early morning hours before the sun rises and starts re-heating the earth, you're getting more gasoline than when you purchase it in the middle of the afternoon. While the volume is the same, the density is different, and in theory you should be able to get better mileage out of the tank that was filled in the early morning.
#31 of 49 Re: 2007 2.5 base automatic, 34 mpg. [pilot1226]
Jun 24, 2008 (6:59 am)
There was a news report on this recently, but I unfortunately don't remember the news source. The bottom line, however, is that whatever gain you get is really minimal and not worth the effort.
#32 of 49 Re: 2007 2.5 base automatic, 34 mpg. [pilot1226]
Jun 24, 2008 (1:06 pm)
The tanks are underground and well insulated from temperature fluctuations.
Jun 25, 2008 (11:29 am)
lower octane fuel doesn't give as much energy per joule compared to plus or premium.
isn't a joule a joule regardless of its anti knock capability????
#34 of 49 Re: joules [cptplt]
Jun 25, 2008 (5:18 pm)
Yes and no. Since there's more resistance to premature detonation, there's more power in the bonds of the hydrocarbon, therefore it releases more energy when the bonds are broken.
It's a minimal gain.
#35 of 49 Re: joules [pilot1226]
Jun 27, 2008 (1:11 am)
Actually from what I remember if your car is tuned to run on regular you will actually get MORE power from a lower octane fuel than from a higher octane fuel. The higher octane is merely there to prevent detonation before the spark plug detonates the fuel due to heat.
Bottom line is unless your car is tuned for premium (turbo cars/supercharged cars, higher performance cars) then you are wasting your $ on premium.
#36 of 49 2008 Outback 2.5i Manual
Jul 06, 2008 (10:25 pm)
I live in Los Angeles and my one-way commute to work is about 13 miles hwy. Round trip, the dash computer says 28.5ish mpg. On a recent 60 mile roundtrip this weekend, trip computer noted 33.6 mpg.
I drive like a grandma (no offense), between 58-63 mpg. I use the cruise control when possible and avoid all the jackrabbit starts at the line. Car only has about 3500 miles on it now, would like to see how mileage changes as we head to our camping spots in the mountains this fall.
#37 of 49 Re: 2008 Outback 2.5i Manual [carlo808]
Jul 07, 2008 (9:01 am)
Grandmothers should not be offended, in fact with current gas prices they should be proud! 33.6 mpg sounds good to me!