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Gas Mileage FAQ

164K views 284 replies 116 participants last post by  JV's WRX 
#1 · (Edited)
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
 
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#55 ·
The interesting thing from that info is the massive drop as you set your cruise for higher speeds. Much more significant than the AC factor, IMO.
 
#57 ·
Definitely. I'm curious if it has to do with speed/gearing or drag more. I remember that cars when I first started driving were calculating highway mileage at 55.... and now it's 65. I'm wondering if they changed gearing to be good at 75 if we could get the same gas mileage, or if it has more to do with the friction than the gearing... Any engineers in here??

I couldn't agree more those are some great tips

Thanks for taking the time
Thanks for taking the time to read it! :D
 
#58 ·
I tend to believe that it has much to do with drag and turbochargers. Once you get the brick cruising down the road at 85mph, it may be that you're now into boost territory. It may be that the way our cars are mapped, there would tend to be a substantial drop in mpg's. It would be nice to have steady-state boost readings to go with those figures provided by cecilia.
 
#59 ·
true. Boost is developed by load though. And gearing has a lot to due with load, as well as the drag. So I'm wondering if 5th/6th gears were changed if we'd be able to stay out of boost more and get decent mileage.
 
#63 ·
That's really interesting! I didn't know that happened...

I know of strictly natural gas vehicles, but I didn't know that there were any hybrids.
 
#64 ·
Maybe because you live in USA and you don't have it here. In Ukraine 50 percent of TAXI drivers use the natural gas and gasoline powered vehicles, they make more money that way....
I wish i can put the natural gas in my impreza, all equipment about 700 $, but it will save you money 1/2 on gas.

YouTube - Honda Civic GX Natural Gas Vehicle Review - Kelley Blue Book
 
#66 · (Edited)
The gas milage thread

Can't believe it's been a full year since i bought the 06 WRX.
10,000 miles. odo showing around 33,500 now...Runs and drives as new..... it averages 23-24 mpg... if i use it to commute i drive from 5000' to 7000' to 6000' and same on the trip home [doh'] and ALWAY'S do at least one full on 1st to second power shift.
Can't complain... One vaca trip we got close to 29mpg fully loaded with AC on...Whats not to love..Just turned 58 on July 15th. :thumbup:
 
#68 ·
Can't believe it's been a full year since i bought the 06 WRX.
10,000 miles. odo showing around 33,500 now...Runs and drives as new..... it averages 23-24 mpg... if i use it to commute i drive from 5000' to 7000' to 6000' and same on the trip home [doh'] and ALWAY'S do at least one full on 1st to second power shift.
Can't complain... One vaca trip we got close to 29mpg fully loaded with AC on...Whats not to love..Just turned 58 on July 15th. :thumbup:
Awesome! :thumbup:

"Fuel Catalysts" I'm really curious about this stuff - haven't tried it and I'm not recommending it. Senior charger forum guys are using it in 89 octane for a 93 octane tune and claiming less FLCK and improved mileage (substantially). I know we're all gun shy of snake oils, but sometimes new things come along.... See post #2 mainly for this thread.

https://www.fordmuscle.com/forums/all-ford-techboard/484599-aces-iv-fuel-catalyst.html#post1484659
Pretty cool
 
#67 ·
#73 ·
good idea. Don't have time now, I will later tonight or tmrw
 
#77 · (Edited)
So, now that my stock 09 wrx has 15K on it I noticed that the mileage had improved considerably, around 5k my average was 20.5+- in mixed Highway/Canyon driving (I live 4 ½ miles up a canyon and the trip to town is 19mi each way).
Recently I noticed that my average mileage on Trip log A was up to 24.5 over the last 1400 miles, mostly the same route. So I did a quick check of what my mileage was in the canyon (14.5mpg) and found that I must be doing fairly well on the highway. Turns out I am, cruise at 64-65 between lights (4 in 14 miles, it is US 30, Scappoose to Portland OR) and I am getting 31.4, averaged over 10 runs, all in the rain, it has been raining quite a bit here lately.
As I don’t believe I have a magic car or a poorly calibrated odometer I assume this is fairly the norm, but I am still quite surprised, happily so, but surprised.
 
#78 ·
I'm not sure how relevant this is to anything that's been said in the past few pages, but it's definitely relevant to gas MPGs.

I did a speech in my communications class a couple days ago on hybrid automobiles vs gasoline, and why hybrids weren't worth the money (at least not yet). I'm sorry this wasn't done with a soobie, but they don't make a hybrid I could compare one of their gas cars to. I found an equation on AOL autos that can find out how much time you have a drive a hybrid car (which costs more than a comparable gas car) until the money you've saved on gas has outweighed the extra price tag on the hybrid. This is really interesting.

Step 1: Take EPA's average miles traveled per year (15000) and divide it by the combined MPG reading for whatever car you choose.

Step 2: Multiply that number by the average gas price (as of a few days ago, it was $2.67/gal). This will yield the car's annual fuel cost.

Step 3: Complete the above equation for both the hybrid and gasoline vehicle.

Step 4: Once you have the annual fuel costs for each car,find the difference between them.

Step 5: Divide the cost difference between each car by the difference in annual fuel costs and you will get the length of time it will take you, driving 15000 miles/year, to make up the extra money paid for a hybrid in gas $.

For the example, I used a 2010 Honda Civic Sedan (about 33 mpg combined) VS a 2010 Honda Civic Hybrid Sedan (about 41 mpg combined).

Gasoline: 15000/33 = 454.54 x 2.67 = $1,213.62
Hybrid: 15000/41 = 365.85 x 2.67 = $976.81
1213.62 - 976.81 = $236.81 difference
Difference in price: $8,145 (according to Honda.com)
8145/236.81 = about 34!

I was like, holy ****! I double checked this, and it holds true. As of now, if you have a 2010 Honda Civic Hybrid, you have to drive 15000 miles a year for 34 years until you've made up that extra $8,145 you paid in saved gas. How about that? I thought that number was more like 6 or 8 years, not 34 years. However, this number (34) will go down considerably if gas goes back up to 4 or 5 bucks a gallon.

But still, I thought that was pretty cool.

Is that Hybrid Worth It?
 
G
#81 · (Edited)
Nice. Probably has crap for power. Chevy is making the Volt too, which is supposed to get like 200mpg or something crazy. At least the diesels still have a little oomph. The Volt will be out sooner than the Volvo too by about 4 years. Nothing like $40,000 for 73hp though!
 
#82 ·
OK, I've got a bit of an issue. I've owned my WRX for a few months now, and the MPGs I've been getting have always been disappointing. I kept telling myself that the more I drove it, the more they would improve because I'd become more familiar with the car. They haven't improved.

So, my question is, is this typical WRX gas mileage? Or is there a problem with my car. As seen in the picture below, I've gotten that many miles out of pretty much a whole tank. 272/16 is pretty much a solid 17 miles per gallon. Is that normal? I really believe I drive pretty responsibly...I've been driving stick since I was 15, and I hardly ever shift too far over 3000 RPMs. Even though I live in Germany and am on the autobahns pretty much every day, I really don't ever exceed 70 MPH (RPMs= 2/300 under 3000). Here are few conditions which could be affecting my car and/or my sense of gas mileage.

1. I stepped up to this from a 1997 Dodge Avenger ES with a 2 liter Mitsubishi Eclipse engine. It got GREAT gas mileage.
2. I've got a short ram intake.
3. It has been frigid lately in Germany. Many days never get above freezing, so the car tends to idle high for a little bit when I first start it in the morning to warm itself up.
4. Because it's cold and snowy, I used the de-froster more than normal.
5. I know for a fact that I have 2 bad O2 sensors...both of which have replacements in the mail somewhere.
6. At that level (in the pic), should my gas light be on?

Am I doing something wrong here? Does my car need to be tuned again (it's got 80k miles)? I'm no Subarologist, so I'm assuming more people on here would know better than me. Thanks for the help!!
 
#83 ·
OK, I've got a bit of an issue. I've owned my WRX for a few months now, and the MPGs I've been getting have always been disappointing. I kept telling myself that the more I drove it, the more they would improve because I'd become more familiar with the car. They haven't improved.



1. I stepped up to this from a 1997 Dodge Avenger ES with a 2 liter Mitsubishi Eclipse engine. It got GREAT gas mileage.
You bought a 2.5L forced induction vehicle for gas mileage? Maybe you should trade it in for a Prius.
2. I've got a short ram intake.
3. It has been frigid lately in Germany. Many days never get above freezing, so the car tends to idle high for a little bit when I first start it in the morning to warm itself up.
The colder air is,the denser it is requiring more fuel.
4. Because it's cold and snowy, I used the de-froster more than normal. Doesn't matter
5. I know for a fact that I have 2 bad O2 sensors...both of which have replacements in the mail somewhere.
Jebus........you think that might be the problem? Install the new sensors then whine about the gas milage
6. At that level (in the pic), should my gas light be on?
All cars are different,this is a given

Am I doing something wrong here? Does my car need to be tuned again (it's got 80k miles)? I'm no Subarologist, so I'm assuming more people on here would know better than me. Thanks for the help!!
You want better gas mileage?Read the whole thread, fix the O2 sensors,get a tune,maybe clean the MAF sensor and make sure your running 36 psi all around on the tires and don't buy a forced induction vehicle for gas milage.
 
#93 ·
I'm in Germany, and they use liters here on the economy. However, on my US forces base we have an American gas station which operates in gallons. But it's $3.07 here for mid-grade! I have no idea why it's so much for us, but cheaper in the states... Still, i'm happy to have an American station, because if I filled up on the economy it would cost me well, well over $100 to top off my car.

They don't even offer reg-grade gas anymore in Europe. Not that I would ever think of putting Reg into my WRX, but I was just surprised that they completely phased it out.

I don't think you quite understand how important those sensors are if they are indeed damaged or functioning improperly
Well, I've definitely got a better idea now. I've done some more reading and I'm hoping that my mpg's will go up significantly once I replace both sensors. If they'd just get here...my only problem is waiting for them! Since I'm in Europe it takes forever for car parts to get over here. :mad:
 
#96 · (Edited)
I was goofing around with my odometer the other day (never realized the center display was tied in till I read the manual) .. over the last 6700mi my average fuel mileage has been 24mpg according to the cars trip computer. Granted I have driven alot of highway and live in a fairly moderate climate (mid atlantic east coast) ... but for the performance of the car I've been pleased with the fuel mileage. I had a feeling it'd be lower given the way I usually drive (I like boost :D). One of these weeks I need to re-set one of the trip meters to see what my mpg is like for a week of normal work n back commute (mostly city, touch of highway). I'm curious to see where it lands but I'm sure that it'll be near/around 18mpg.

to the op;
dont forget that higher cruise speeds will also drop your fuel mileage. Make sure your tire pressure is where its supposed to be. If you haven't checked it since it got cold.. it very well could be off (not necessarily off enough to throw the tpms but off enough to toss your mpg. You're already on to the o2 sensor importance.. what oil do you use? what air filter?

edit:

now with just over 10k on the car, the average fuel mileage is still mid 23mpg range. I am quite pleased that a car with this much snot and awd gets the fuel mileage it does. Granted, its no fuel sipper.. but for what it does.. it does it pretty efficiently ;)
 
#104 ·
Your best mileage will be the slowest speed in 5th gear w/o lugging the engine.

Realistically just stick to the speed limit and you'll be amazed at the mpg you can get. We take our wife's honda civic on vacations and we can get 35-40mpg easily just by not speeding.

If you're really that short on money and worried about mpg you bought the wrong car. :(
 
#107 ·
I drive mostly highway (about 15 mile commute to work) which does include a little bit of city driving (> 1 mile). I re-set my trip every time I get gas and I generally average 25.8 - 26.7 according to the computer's MPG. I generally drive softly to and from work and almost always have fun around the weekend.

Overall, I'm pleased.
 
#109 ·
Sinister said:
Just like lengthwise mentioned, stick to your lowest speed that is efficient in 5th gear. (A.K.A... use the least amount of throttle at the lowest RPM you can still maintain speed at).

It'll probably be about 55-60mph.
55mph nets me my best highway mileage
 
#111 ·
+1
I've had a couple of cars with trip computers; measuring at the pump was always much more accurate.

I've always gotten pretty great mileage out of my WRX's for what they were...but not so much anymore. Hybrid engine+18G+cold weather seems to have put a nice trough in my mpg graph. :rotfl:
 
#112 ·
yeah... that's a bummer. I've heard that without the avcs that the 2.5L is a hog :(
 
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