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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; Last night, gas prices went up 5 cents in my area... 14 cents in a week. It's that time of ...

  1. #1
    UnBanned Sinister's Avatar
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    Gas Mileage FAQ

    Last night, gas prices went up 5 cents in my area... 14 cents in a week. It's that time of year. So I figured I'd do some testing to see how to increase mileage

    Very first thing: I have a 2006 WRX with a VF39 and tune. Your car will not get the same mileage numbers as me, but this FAQ will still help you increase your gas mileage.


    There are several different parts of gas mileage. Driving style, efficiency, rolling resistance, weight, aerodynamics, and accessory usage.

    I'll start with Accessory Usage. Obviously you can't get something for nothing. Every accessory you use in your car is run on gas. Whether it's electrical, mechanical, or a combination. It's simple, the more accessories that you use, the worse the gas mileage. The further away from peak torque you are, the bigger the effect.

    I found that using my Air Conditioner while cruising the highway at 55mph actually lowered my instantaneous gas mileage approximately 8mpg!! The engine has more load on it when it's operating the A/C. When that load is in a place where you're not developing power/torque (ie cruising rpms 18-2800rpm) then it's going to make that much more of a difference.

    I do not have an upgraded stereo, but if you have an aftermarket sound system with a large amplifier, you are losing a fair amount of gas mileage from the power that's required for the amp. The power comes from the alternator, which is on the accessory pulley system, and this causes more load on the engine decreasing mileage.

    It's that way with anything you use in your car. But mostly we are concerned with Air Conditioning. If you can stand to have your windows open this summer and the A/C off, it will save you a large amount of gas mileage.


    Next let's talk about Aerodynamics. The majority of us aren't going to be concerned about this. But Aerodynamics is basically how smoothly your car moves through the air without causing resistance. The less aerodynamic your car is, the more air resistance it has (it "catches" the air).

    Everyone that has Bike Racks, Ski/Snowboard racks, or a Camping "Pod", and it's not being used: Take it off! These create a large amount of resistance, and only take a few minutes to install for your next mountain biking/camping/ski trip. The more resistance you have, the harder the car has to work to maintain the same speed. That's why race cars are designed with so little resistance. The lower your aerodynamic efficiency, the worse your gas mileage.

    Drafting is dangerous, and I don't do it, or recommend it. It will damage your front end and windshield with rocks, lessen the amount of time you have to react to a vehicle slowing in front of you, cut the distance you have to stop, and you will be out of the trucker's side view mirrors. But because they create a "slip stream" through the air, if you're in that slip stream, you will not have as much air resistance, and you will incur increased mileage. I don't recommend this, as it's disrespectful to the trucker, and dangerous to you and the cars around you.

    Another note to this is speed.
    ~~ Resistance is not linear. The faster you are going, then you have exponentially more resistance. So going 100 mph is going to be worse gas mileage than 65 mph.~~


    The next topic is Weight. It's simple. The heavier your car is, the harder the engine has to work at the same speed. Removing my spare tire actually gained me 2mpg in one test. So if you're a pack-rat that has a trunk full of junk, take the time to remove it (especially the heavy stuff!) and save yourself a few bucks the next fill up!

    On the same note is Rotational Inertia. Rotational Inertia is roughly described as weight moving in a circular motion. Wheels, Tires, driveshafts, etc... all have rotational inertia. The higher it is, the harder the car has to work. So a pair of lightweight wheels will have a lower inertia than your stock wheels. They will help you gain gas mileage. A lightweight flywheel/crank pulley will (in theory) help you increase your gas mileage. Now the gas mileage you gain will not be as much at a steady speed, but while accelerating, and decelerating (ie city driving, or stop/go traffic) you should see noticeable gains.


    Rolling Resistance is the resistance of the tires against the ground. Tire pressure affects the shape of the tread pattern, and the size of the contact patch (the amount of tire on the ground). Increasing tire pressure distorts the tread pattern so that the tread isn't smoothly contacting the ground. Conversely, if you lower your tire pressure, you have increased friction due to increased contact patch. Safety is also an issue. Higher tire pressure means an increased braking distance, and a lower tire pressure means softer side walls which in turn means steering is very negatively affected. Tread wear is also affected negatively with a higher, or lower tire pressure than what is recommended. For safety, and mileage, make sure you follow the manufacturer's guidelines as to what PSI your tires should be inflated to.


    On to Engine Efficiency. The more efficiently you use your fuel, the more power you get out of each stroke, therefore the less fuel you need to use to get the same power. This is seen in a quality tune. You car runs rich stock, and on off the shelf tunes. This means there is more gas injected into the cylinder than is necessary. This is done for safety reasons, and because each engine is slightly different, the factory doesn't want to tune each car's engine. Now when you get a custom tune, your car is "taught" to use fuel in a more efficient manner. Yes, this is done for power, but a great side effect is the gas mileage. As long as you are driving appropriately, you will be saving gas with a good tune!

    You're all good WRX owners so I'm sure you know that you absolutely have to use 91+ octane. If you don't use the required octane, your engine's timing will retard, and you will lose mileage. You will not save any money by using the cheaper lower octane gasolines.

    (Side Note: CELs can put the car into "Limp Mode" or indicate that certain sensors aren't functioning properly. This will brutalize your mileage)


    And finally... Driving Habits. If you improve upon all of the other aspects listed above, but every time you leave a light you give the throttle a firm push to the floor, then you'll see only negligible differences in mileage.

    There are several factors to consider while driving. Throttle Position, Engine Load, Boost and RPM.

    Throttle Position is what percentage of your throttle is down, between 8% (idle) and 100% (full throttle) Because we are not diesels we have a throttle plate, and while the car is running it has to be partially open. My car idles between 8-9% throttle, and I didn't go past 20% throttle while I've been experimenting. Just pay attention to is how far down your pushing the pedal.

    Engine Load is how much weight the engine is actually pushing. When you are going up hill, the engine fights the weight of the car, and gravity. This creates more engine load. When you're going downhill, gravity assists and you have less engine load. It's the same with vehicle weight. The more weight in the car, the more engine load. Unfortunately, terrain is unchangeable, but just because you're on a hill, that doesn't mean you need to floor it. Just push the accelerator down until you're at a steady speed.

    A side note on Engine Load: Oil reduces friction in your engine, which consequently reduces effort/heat/load. Be sure to change your oil regularly with a reliable proven brand. No-name brands may not meet the required GF rating. I personally use a 5w-30 full synthetic, but when i lived in the desert I used a 10w-40 full synthetic. 02-05 required GF-3/4/5, and 06+ require GF-4/5

    Boost. We all know what it is. It's the amount of air pressure the turbo is creating. Well, the more air there is being compressed in the cylinders, the the more fuel must be added. So if we stay out of boost, then you won't consume as much fuel. Simple as that.

    Your RPM's effect on gas mileage has a lot of falsities to it. There are a few things you must consider. The higher the RPM, then the more times per minute each piston is pumping. So if you are at 4k RPM, vs 2k RPM with the same throttle position, same load, same boost... (we're only talking RPM) then the 4k RPM will consume more fuel per minute, just on the number of times the cylinders fire per minute. So you'd think "The lower the RPM the better the gas mileage?" Not necessarily. There's a fine line. When you get too low, there isn't enough torque, and you have to increase throttle position to maintain speed. That's why you don't drive 40mph in 5th or 6th gears. So you want to drive at the lowest RPM with the lowest throttle position in the gear that best matches your speed.

    Every car is different, but I usually stay between 1800 and 2800 RPM to get the best mileage. On my vehicle, 1800 RPM matches the speed to 10x the gear. What I mean is, in 5th at 50mph I'm at 1800rpm, 4th at 40 at 1800, 3rd at 30 I'm at 1800, etc.

    Another consideration in your driving habits is Newton's First Law of Physics: "An object in motion tends to stay in motion, and an object at rest tends to stay at rest unless acted upon by outside force." Basically it takes energy to start you from a stop. More than if you accelerate while you're moving. If you attempt to coast into stop lights, etc. then you will also save gas. Don't do anything illegal (run stop signs) or unsafe just to save gas though!

    Idling excessively is also a gas mileage waster. Sitting with your engine on for extended periods of time is a waste. If you stop at a convenience store, or drop by your house to grab something... Turn your car off! You'll save gas and money in the long run!


    Any questions, comments, or corrections. Please post!

    Thanks,
    Kevin
    Last edited by Sinister; 04-14-2010 at 03:29 PM.
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  3. #2
    Not Asian Ying's Avatar
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    I would add tire pressure to your piece on gas mileage, but I think you've got a pretty good write up there.

    Something I was curious about related to boost, RPM and throttle. If you're at low RPMs (40 in 4th, 50 in 5th per your write-up) and begin on an incline where the engine begins bogging, is it better to increase throttle which increases boost to remain at low RPMs, or downshift for higher RPMs and use less throttle?

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    Another gas saving tip is to avoid coming to a complete stop when possible. Slowing down and picking up speed while you're already in motion can definitely have an impact so just look ahead to see if traffic/traffic lights will allow you to do this.

    Don't blame me though if you do this going through stop signs and get a ticket.

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    You forgot one very important concept in maximizing fuel efficiency...something that Toyota absolutely banked on: idle time.

    Any idling at stoplights, and even stop signs (for those couple seconds) wastes gas. So what do you do? You turn the engine off. That's where the Prius gets the high gas mileage. Anytime the engine is not needed for motive power, the car shuts it off automatically. It's a pretty simple concept really...it's just automated. I just wish GM or Ford came up with it first, but that's another story.

    Of course, this isn't exactly practical with a Suby. It puts unnecessary wear on the starter and battery. But I have done it with my truck for those long turning lane lights when gas was at its peak. Verified by my ScanGauge (in truck), it did help because any high MPG I aquired by moving would quickly be reduced by idling, even for a couple of seconds. This applies especially on short trips where the average has more highs and lows rather than constants.

    So however you want to do it, consider idle time.

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    Quote Originally Posted by Bebop View Post
    Don't blame me though if you do this going through stop signs and get a ticket.
    Oh yeah! Stop signs waste gas! For awhile when gas was high(er), I was having conspiracy theories that stop signs were installed by Big Gov to increase gas consumption, especially on roads that would benefit only from a two-way stop rather than a three or four-way stop. Crazy I know.

    But all in all, he's right.

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    Administrator RayfieldsWRX's Avatar
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    Great writeup, Kevin.
    My experience has been that your final speed also makes an impact on mileage. The energy required to maintain a given speed of a vehicle increases in a non-linear way as you go faster. If part of your commute is on a highway, you can set the cruise for a few clicks lower than you might normally, and you will probably see a difference at the pump.

    I realize that this may be hard for the typical, youthful ClubWRX demographic to deal with; for me it's no big deal to go just a couple of mph above the posted limit. Whether you're going 57 or 70, driving on the highway is boring. About 20 miles of my commute is highway; if I go 57 instead of 70 (where the limit is posted 55) it adds a grand total of 3 minutes to it. That's fine..more podcast/music time.
    Last edited by RayfieldsWRX; 05-14-2009 at 07:40 AM.
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    Master Baiter EJ257's Avatar
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    Great write-up

    Quote Originally Posted by RayfieldsWRX
    I realize that this may be hard for the typical, youthful ClubWRX demographic to deal with; for me it's no big deal to go just a couple of mph above the posted limit. Whether you're going 57 or 70, driving on the highway is boring. About 20 miles of my commute is highway; if I go 57 instead of 70 (where the limit is posted 55) it adds a grand total of 3 minutes to it. That's fine..more podcast/music time.
    I wholeheartedly agree with this.

    Meanwhile, when you get pulled over for doing 70 (15 over in NY can easily get you nabbed), expect to add an extra 20-30 minutes to your trip while you wait for the officer to run your information, and write you a ticket
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    UnBanned Sinister's Avatar
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    First off, thanks for all the positive feedback!!

    I did forget some things, but notice what time I posted this



    Quote Originally Posted by Ying View Post
    I would add tire pressure to your piece on gas mileage, but I think you've got a pretty good write up there.

    Good add! Thanks!

    Something I was curious about related to boost, RPM and throttle. If you're at low RPMs (40 in 4th, 50 in 5th per your write-up) and begin on an incline where the engine begins bogging, is it better to increase throttle which increases boost to remain at low RPMs, or downshift for higher RPMs and use less throttle?

    This is a good question. My personal experience has been... if you are in a low RPM, and you're more than 1/4 throttle, then you need to shift to a lower gear! Just remember the basic equation: Low Throttle + Low Boost + Low RPM + Low Engine Load = Great Gas Mileage! If you're in the extremes on one of these things (ie 100% throttle) then you probably need to change a gear to try and fit the equation!
    Quote Originally Posted by Bebop View Post
    Another gas saving tip is to avoid coming to a complete stop when possible. Slowing down and picking up speed while you're already in motion can definitely have an impact so just look ahead to see if traffic/traffic lights will allow you to do this.

    I actually meant to add this. While pulling up to a stop light, I coast into it. It's one of Newton's Basic Laws: An object in motion tends to remain in motion, and object at rest tends to stay at rest. Good call though, I'll add it!
    Quote Originally Posted by 2:43AM View Post
    You forgot one very important concept in maximizing fuel efficiency...something that Toyota absolutely banked on: idle time.

    Any idling at stoplights, and even stop signs (for those couple seconds) wastes gas. So what do you do? You turn the engine off. That's where the Prius gets the high gas mileage. Anytime the engine is not needed for motive power, the car shuts it off automatically. It's a pretty simple concept really...it's just automated. I just wish GM or Ford came up with it first, but that's another story.

    Of course, this isn't exactly practical with a Suby. It puts unnecessary wear on the starter and battery. But I have done it with my truck for those long turning lane lights when gas was at its peak. Verified by my ScanGauge (in truck), it did help because any high MPG I aquired by moving would quickly be reduced by idling, even for a couple of seconds. This applies especially on short trips where the average has more highs and lows rather than constants.

    So however you want to do it, consider idle time.

    Another good point. I'll add something to do with this
    Quote Originally Posted by RayfieldsWRX View Post
    My experience has been that your final speed also makes an impact on mileage. The energy required to maintain a given speed of a vehicle increases in a non-linear way as you go faster. If part of your commute is on a highway, you can set the cruise for a few clicks lower than you might normally, and you will probably see a difference at the pump.

    I realize that this may be hard for the typical, youthful ClubWRX demographic to deal with; for me it's no big deal to go just a couple of mph above the posted limit. Whether you're going 57 or 70, driving on the highway is boring. About 20 miles of my commute is highway; if I go 57 instead of 70 (where the limit is posted 55) it adds a grand total of 3 minutes to it. That's fine..more podcast/music time.



    I did have this Ray, but maybe I can emphasize this a bit more, and in a more descriptive fashion.

    Another note to this is speed. Resistance is not linear. The faster you are going, then you have exponentially more resistance. So going 100 mph is going to be worse gas mileage than 65 mph.


    I don't have the time right now, but I'll add all of these suggestions later today.

    Thanks for the read guys, and all of the comments/feedback. Hope I helped at least one person!

    (Just a side note, I'm at 70miles, and I'm sitting on the "F" line still)
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    Quote Originally Posted by Bebop View Post
    Another gas saving tip is to avoid coming to a complete stop when possible. Slowing down and picking up speed while you're already in motion can definitely have an impact so just look ahead to see if traffic/traffic lights will allow you to do this.

    Don't blame me though if you do this going through stop signs and get a ticket.
    Another tip that follows this is to keep distance between you and the person in front of you. Especially if they keep hitting the breaks randomly or seem to have trouble maintaining speed. Don't let bad habits of another driver burden you.

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    DUB
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    Quote Originally Posted by 2:43AM View Post
    ...That's where the Prius gets the high gas mileage...
    Yeah, that and the fact that part of the load is shared with an electric motor.

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    Quote Originally Posted by DUB View Post
    Yeah, that and the fact that part of the load is shared with an electric motor.
    Yes, that plays a part, but power in almost equals power out. Regenerative braking also helps overcome that loss. The Prius's MPG would not be as great if the engine stayed on 100%.

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    Quote Originally Posted by 2:43AM View Post
    Yes, that plays a part, but power in almost equals power out. Regenerative braking also helps overcome that loss. The Prius's MPG would not be as great if the engine stayed on 100%.
    From my knowledge, it's all electric unless there is a lot of load on the engine. Correct?
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    DUB
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    Quote Originally Posted by Sinister View Post
    From my knowledge, it's all electric unless there is a lot of load on the engine. Correct?
    correct. except "a lot of load" on the engine is like 4 mph.

    stop and go traffic, parking lot - that can be accomplished on the electric motor alone. as soon as you accelerate away from a stop sign at a normal pace the gas motor kicks in.

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    -You're both right. Another JAP-to-USA conversion blunder is that the US Prius does not have the "electric only" switch, where the driver can push the button and run the car on battery basically until the battery gets too low. Then the gas engine kicks in and replenishes it. Pretty lame how they omitted that for US markets.

    But overall, power isn't created magically. Whether you're driving around at 4mph or 40mph, the energy it takes to get you there has to either come from the battery or the engine. That is unless your start and finish destinations are both downhill, both ways...which of course is impossible in this universe.

    However, we're digressing from the original thread topic.

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    UnBanned Sinister's Avatar
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    UPDATED!

    Mods, is this worth a sticky?
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