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This is a discussion on Gas Mileage FAQ within the Everyday Impreza Talk forums, part of the Community - Meet other Enthusiasts category; Stickied! You're right Kevin, you did mention speed; I just didn't look at your aerodynamics section that closely during my ...

  1. #16
    Administrator RayfieldsWRX's Avatar
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    Stickied!

    You're right Kevin, you did mention speed; I just didn't look at your aerodynamics section that closely during my first read through.
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  3. #17
    UnBanned Sinister's Avatar
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    I made over 350 on a tank.

    So I decided to drive like a normal WRX owner for a tank, and got 270.

    So 80 miles on a tank is a pretty big difference!

    With gas prices up to 2.80 in some places in Colorado... I implore some of you to re-read my FAQ!
    Last edited by Sinister; 05-26-2009 at 11:58 AM.
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  4. #18
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    Great write-up Kevin I enjoyed reading this.
    Last edited by mycologist; 05-27-2009 at 08:24 AM.
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    Sup all. I've got some suggestions to add to this wonderful thread. I think you should add oil viscosity to the engine load topic, add something about the recommended grade gas in the tank, add drafting to aerodynamics, and edit the AC usage tidbit.

    I suggest adding oil viscosity to the engine load section of your post because I noticed that when I switched from mobil 1 10w-30 to mobil 1 5w-30, I gained about 2 mpg. I believe this is from the fact the more viscous oil provides more resistance than the less viscous. I think this is referred to as windage?

    The car that I currently have is a 91 legacy N/A. Since it is an N/A the recommended fuel is 87 octane. I've put 89 and 91 octane and have noticed no difference in mpg so that was a waste of money. However if this were a turbo car (i'm getting a rex in june ) the car would lose mpg when using lower grade gas, not to mention loss of performance.

    The drafting bit is for those who are willing to sacrifice their paint and those that have the stickiest tires for modest increases in mpg. Driving by yourself on the highway, you will have air resistance but drafting removes a good portion of the air resistance. The sticky tires are to help reduce stopping distance since drafting requires you to get closer to a car than you are normally comfortable with. This is a clip from the mythbusters tv show. But like they say it has it's risks. YouTube - Drafting Semi I sometimes draft behind semi's at a reasonable distance (~20ft) but since they kick up rocks, it's something I'm going to do less frequently and with greater distance when I get my rex.

    With the AC topic, I agree that it uses up fuel and is parasitic to fuel economy but at highway speeds wouldn't opening your windows add more air resistance than using AC? I was trying to find the mythbusters clip on youtube that showed the differences in mpg with the windows down at highway speeds vs windows up and AC on at the same speed but I couldn't find it. From what I remember, they used a suburban and found that at speeds below 45-50 mph windows would be more mpg friendly but at or above 50-55 mph the AC was more mpg friendly. Since our cars are more aerodynamic than a brick on wheels I think we can increase the speed to 50-55 mph with the windows down as well as 55-60 mph before we have to turn on the AC.

    One last thing. Sorry for such a long post. When you mentioned coasting to a stop, do you coast in a high gear or do you coast in neutral? I think it would be more fuel efficient to coast in a gear than to coast in neutral because if the car is in neutral then the car is just idling. If it's coasting to a stop in a high gear then does the ecu know this and decrease fuel supply to the cylinders or something?

    Excellent write up btw

  6. #20
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    Quote Originally Posted by turbo flat4 View Post
    With the AC topic, I agree that it uses up fuel and is parasitic to fuel economy but at highway speeds wouldn't opening your windows add more air resistance than using AC? I was trying to find the mythbusters clip on youtube that showed the differences in mpg with the windows down at highway speeds vs windows up and AC on at the same speed but I couldn't find it. From what I remember, they used a suburban and found that at speeds below 45-50 mph windows would be more mpg friendly but at or above 50-55 mph the AC was more mpg friendly. Since our cars are more aerodynamic than a brick on wheels I think we can increase the speed to 50-55 mph with the windows down as well as 55-60 mph before we have to turn on the AC.
    You know... I thought the same exact thing, and looked at a lot of information in my quest for gas mileage wisdom... but it really depends on the car. I didn't test all 4 windows, but at 65mph with the front 2 windows down, and AC off... I got better gas mileage than all 4 windows up, and the AC on.

    I did see the same gas mileage mythbusters too. They had a truck with a tonneau cover and one with no tail gate, etc as well in that episode, right? Good episode.

    I did this multiple times over the same stretch of road with the cruise control on. So the results weren't skewed.


    One last thing. Sorry for such a long post. When you mentioned coasting to a stop, do you coast in a high gear or do you coast in neutral? I think it would be more fuel efficient to coast in a gear than to coast in neutral because if the car is in neutral then the car is just idling. If it's coasting to a stop in a high gear then does the ecu know this and decrease fuel supply to the cylinders or something?
    Most cars are that way. I looked into that as well. many Diesels even completely cut the fuel, and use the cylinder compressions to slow the vehicle.

    The WRX is not that way though. I actually logged the injector duty cycles while coasting to a stop in neutral, vs coasting to a stop in gear. The neutral coast had a lower injector duty cycle (meaning how much fuel was being injected in a percentage of the maximum that the injector can handle) than coasting in a gear. This IS a safety hazard to be in neutral though, because you cannot accelerate out of the way of a car that's about to hit you, or to avoid a car coming into your lane. I don't recommend using neutral while coasting at all, but I will say that it does help gas mileage.




    I'm not a fan of drafting, as it ruins front ends, windshields, and if you're close enough to draft... the trucker can't see you in his mirror. I will mention something to the effect of what you wrote, but I will mention the dangers, and why I don't agree with, or recommend it.



    The other information has some really good bits and pieces (the viscosity / octane) and I'll be sure to edit this later and add the information.



    Thanks for adding all of this, and taking the time to read the whole article! It was one hell of a time consuming post to validate on the highway and city... and took a while to write, so I'm glad at least a few people are enjoying it!

    (updated the info you listed. Thank you!)
    -Kevin
    Last edited by Sinister; 05-26-2009 at 04:36 PM.
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  7. #21
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    Quote Originally Posted by tmann05 View Post
    Great write-up Kevin I enjoyed reading this.
    I'm glad you enjoyed reading this!
    Last edited by mycologist; 05-27-2009 at 08:24 AM.
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    On topic: I am really trying hard to squeeze out a couple more MPG's on my current tank. We'll see what happens. So far, I am at 165mi at about 1/2 tank. But the 2nd half always goes the fastest! I'm sure I will fall short.

    Also, summer blended gas around my area really sucks. There is more ethanol than winter blends, up to 10%. I'm sure that affects my mileage also. You may want to mention that in your thread too.
    Last edited by mycologist; 05-27-2009 at 08:19 AM.

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    You know... I thought the same exact thing, and looked at a lot of information in my quest for gas mileage wisdom... but it really depends on the car. I didn't test all 4 windows, but at 65mph with the front 2 windows down, and AC off... I got better gas mileage than all 4 windows up, and the AC on.

    I did see the same gas mileage mythbusters too. They had a truck with a tonneau cover and one with no tail gate, etc as well in that episode, right? Good episode.

    I did this multiple times over the same stretch of road with the cruise control on. So the results weren't skewed.
    That is really interesting. I would think that because the rear half of the car has no place the air can go it would act like a mini parachute creating drag. Perhaps it would if all 4 windows were down and maybe with increased speed.

    There is a different clip from mythbusters that I'm looking for. They used a closed track and hooked up the white suburban to a laptop from it's OBDII connector to monitor it's mpg. They made a mistake for not accounting for changes in speed from the first episode so they had to revisit it and correct the results. But yeah since it's a suburban, it's huge and not really aerodynamic whereas our subaru's are more compact in comparison.

    The WRX is not that way though. I actually logged the injector duty cycles while coasting to a stop in neutral, vs coasting to a stop in gear. The neutral coast had a lower injector duty cycle (meaning how much fuel was being injected in a percentage of the maximum that the injector can handle) than coasting in a gear. This IS a safety hazard to be in neutral though, because you cannot accelerate out of the way of a car that's about to hit you, or to avoid a car coming into your lane. I don't recommend using neutral while coasting at all, but I will say that it does help gas mileage.
    I was hoping you had info on this and you did. YAY!! Unfortunately it isn't the results I had hoped for. O well. I always coast in a gear.

    For the drafting part I knew I forgot to add a disclaimer saying that should you do this, you realize the dangers associated with driving like this and this forum takes no responsibility for any accident that you may incurred. You, the driver, assume all risk. blah blah blah. I really hope nobody is that stupid/desperate to get within 10 feet behind a truck.

    This was a really good post, I enjoyed reading it and I hope other people can contribute

    Thanks

  10. #24
    UnBanned Sinister's Avatar
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    Quote Originally Posted by 2:43AM View Post
    On topic: I am really trying hard to squeeze out a couple more MPG's on my current tank. We'll see what happens. So far, I am at 165mi at about 1/2 tank. But the 2nd half always goes the fastest! I'm sure I will fall short. Let me know how it goes!

    Also, summer blended gas around my area really sucks. There is more ethanol than winter blends, up to 10%. I'm sure that affects my mileage also. You may want to mention that in your thread too.

    You know... I had thought about that. I didn't mention it because I can't get any definitive proof on this. Using E85 you have to inject more fuel into the cylinder to remain stoich. Now with 91 octane that's 10% ethanol, I'm wondering if our computers are able to calculate that the AFR is lean, so they inject more fuel to compensate, and you end up with worse mileage... or is there so little ethanol being used that there is no more fuel that's injected, and we run just slightly lean from the ethanol, but the timing is still advanced because we are still at 91 octane. I can't find any proof to support either idea, so I just said screw it... it's pretty damned comprehensive as it is! lol
    Quote Originally Posted by turbo flat4 View Post
    That is really interesting. I would think that because the rear half of the car has no place the air can go it would act like a mini parachute creating drag. Perhaps it would if all 4 windows were down and maybe with increased speed.

    I don't know! But at 65, I can tell you that I lost about 20% mileage with the AC on, and about 10% with the windows down. I used the AccessPORT to look at their mileage meter to figure this out. I also looked at injector duty

    There is a different clip from mythbusters that I'm looking for. They used a closed track and hooked up the white suburban to a laptop from it's OBDII connector to monitor it's mpg. They made a mistake for not accounting for changes in speed from the first episode so they had to revisit it and correct the results. But yeah since it's a suburban, it's huge and not really aerodynamic whereas our subaru's are more compact in comparison.

    I'm thinking that the biggest reason why AC is so negative at highway speeds in our car is the ratio of load vs torque. If we were cruising in boost, then the torque is much greater, and so the load/torque ratio is much lower. But since we cruise at 2200 rpm where there is so little torque that the engine has to work a lot harder to maintain speed and support the AC.

    Also, because resistance is not linear, I'll bet if we were cruising at 85mph, that the resistance would be a lot greater from windows being down, and the load would be closer to the top of our torque curve so it would make more sense to cruise with AC.

    I don't drive 85 on the highway too often, so if anyone does... feel free to let me know the differences!


    This was a really good post, I enjoyed reading it and I hope other people can contribute

    Thanks a lot man.. I really do appreciate it!
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    Administrator RayfieldsWRX's Avatar
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    Quote Originally Posted by Sinister View Post
    No big deal, he just made the mistake of screwing with the old fogie

    Those old people... they're just plain mean!
    Cretin.

    I know it seems mean, but years doing this job have taught me that n00bs that respond that way to a politely-worded warning are not going to be compatible with our forum. I no longer have the patience to verbally dance with them for an hour... I ban'em and let Odin sort them out.
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    Since this is a sticky I cleaned the thread. If someone could hard delete those posts that would be better.
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    Hardleted.
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    Quote Originally Posted by Sinister View Post
    Thanks for adding all of this, and taking the time to read the whole article! It was one hell of a time consuming post to validate on the highway and city... and took a while to write, so I'm glad at least a few people are enjoying it!

    (updated the info you listed. Thank you!)
    -Kevin
    Really enjoyed this your post, thank! I'm not actually a WRX owner, but I've been on ClubWRX reading and reading in preparation for my WRX purchase at some point down the road. It will be a black hatchback, non-sti, probably '09. I actually was looking at a WV diesel for a while because of fuel economy, but man, I love the WRX, especially the new hatch. And AWD is important, too, as a NE driver.
    Last edited by BigGuns11; 06-05-2009 at 07:25 AM.

  15. #29
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    Quote Originally Posted by BigGuns11 View Post
    Really enjoyed this your post, thank! I'm not actually a WRX owner, but I've been on ClubWRX reading and reading in preparation for my WRX purchase at some point down the road. It will be a black hatchback, non-sti, probably '09. I actually was looking at a WV diesel for a while because of fuel economy, but man, I love the WRX, especially the new hatch. And AWD is important, too, as a NE driver.
    I appreciate it!

    Definitely drive both before you make the decision... I think you'll make the right decision
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  16. #30
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    I have one of those freakin' cars that gets exactly the same gas mileage no matter how a drive. Drive like a grandma and don't go over 2500 RPM with almost all highway miles, 18-19 mpg. Easy going city driving, 18-19 mpg. Floor it from light to light (I don't actually do it, just for comparison purposes), 18-19 mpg.

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