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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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#154 ·
Time for me to come clean. I'm a noob, and not just to my four-day-old (to me) WRX, but to like, everything mechanical. Frankly, I'm impressed by roll-on deodorant, so consider this when reading any of my posts.

I've had my 2005 WRX for about four days, now. I bought it bone stock from a used dealer with 64K miles. I did not do a car fax on it (I know, I know).

So, of course, we have to factor in that I'm used to operating three, and only three, manual transmission vehicles...a Honda 750 Aero, a Honda 599 Hornet, and an '03 SVT Focus.

Why am I only getting 18.7mpg? Doesn't this seem rather low? I'll admit that I had to spool it out the first two days (I mean, c'mon, you don't expect me to drive like Pop-pop, do you?), but I filled up to half yesterday, drove about 70 miles (about half highway, half city) today, and lost a quarter of a tank. A QUARTER OF A TANK.

Yes, yes, it's going to a mechanic this weekend for a once-over. Tire pressure is a tad low, but nothing remarkable. No CEL's. Working on a slightly rough idle that could just be in my head.

I'm trying to figure out if it's the car, or if it's me. Tomorrow, I am going to make a conscious effort to stick to the x10 rule (Shift to 4th at 40mph, etc), and see if I notice a difference. I just wanted feedback on mileage. Seems like others are getting a lot better mileage.

Thanks in advance for any thoughts.
 
#156 ·
My '06 consistently got 21-23 around town, and 30-32 on the highway --- both when driven conservatively for fuel economy. My '09 got 19.5-21.0 around town and 28.5-30.0 on the highway when driven for economy.

FWIW related to one of the earlier posts - the hybrids' (Prius, et al) ability to get better in town is only partially due to shutting the engine off when stopped. The largest part of their efficiency gains comes from capturing the energy lost as waste heat during braking. When the brakes are applied in a hybrid, the electric motor(s) that augment the engine to power the car are turned into generators --- the harder the brake is applied, the more energy the generators return to the car's battery to be stored for later use. Later as in accelerating away from the stop sign/light. Only a small amount of fuel is saved by not idling the engine at the light. A much great amount is saved accelerating away from the light --- because the energy stored in the battery while stopping the car can now be used to power the car under acceleration.
 
#161 ·
:rotfl: @ your screen name.


A brand new engine's tolerances are usually very tight. Your first few thousand miles will not be your best gas mileage... plus you and your car need to be better acquainted ;) so you know how to shift. Shifting at a low RPM isn't necessarily giving you the best gas mileage. You'll figure it out as you drive it more.
 
#164 ·
This cold weather has been getting me. I have a 4.5 mile commute to work (lots of hills and stop signs/lights) and with letting my car heat up before i drive, been getting ~17-18mpg

Drove up to Erie PA from Pittsburgh the other weekend and was going about 75mph the whole way there and got just over 25mpg (just under 25 on the way back driving 65 and had a ca FILLED with stuff)

I was happy


it's funny about the MPG i'm getting now though because it's such a short commute so my tank of gas still lasts me a long while :)
 
#165 ·
2011 WRX owners MPG ?

I was wondering if there are any 2011 WRX owners here and if they could post their real world MPG. I"m in the process of ordering a new 2011 WRX and the MPG is the only thing that I"m not 100% comfortably with regarding the WRX. I have read a lot of reviews online and they all said that the car is amazing that the only big down side is the MPG. I'm not expecting to get the MPG of a Civic but if the 2011 WRX would get at least the MPG that the sticker says 19/25 I would be happy with it and order one this weekend.
Sorry for the rant guys my first post and probably the one that will help me the most to spend my $ wisely on a new car.
Thanks a lot for any input
V
 
#166 ·
I was wondering if there are any 2011 WRX owners here and if they could post their real world MPG. I"m in the process of ordering a new 2011 WRX and the MPG is the only thing that I"m not 100% comfortably with regarding the WRX. I have read a lot of reviews online and they all said that the car is amazing that the only big down side is the MPG. I'm not expecting to get the MPG of a Civic but if the 2011 WRX would get at least the MPG that the sticker says 19/25 I would be happy with it and order one this weekend.
Sorry for the rant guys my first post and probably the one that will help me the most to spend my $ wisely on a new car.
Thanks a lot for any input
V

If your driving it with your foot in the transmission expect around 17mpg and if you drive it lightly then you'll get about 22mpg... I average about 22 and when I go out and drive it hard I get about 18ish.... the most I've got was 28, so it all depends on your driving style...
 
#167 ·
I was wondering if there are any 2011 WRX owners here and if they could post their real world MPG. I"m in the process of ordering a new 2011 WRX and the MPG is the only thing that I"m not 100% comfortably with regarding the WRX. I have read a lot of reviews online and they all said that the car is amazing that the only big down side is the MPG. I'm not expecting to get the MPG of a Civic but if the 2011 WRX would get at least the MPG that the sticker says 19/25 I would be happy with it and order one this weekend.
Sorry for the rant guys my first post and probably the one that will help me the most to spend my $ wisely on a new car.
Thanks a lot for any input
V
Thanks for the input 06scoobyrex. I have a really light foot so I guess that shouldn't be a problem. I see that User ID says 06 I was wondering if there are any 11 owners and if their average is anywhere near yours. Probably not, they are all excited and to happy with their brand new cars to spend any time on the these boards for a while :thumbup:
 
#172 ·
I was wondering if there are any 2011 WRX owners here and if they could post their real world MPG. I"m in the process of ordering a new 2011 WRX and the MPG is the only thing that I"m not 100% comfortably with regarding the WRX. I have read a lot of reviews online and they all said that the car is amazing that the only big down side is the MPG. I'm not expecting to get the MPG of a Civic but if the 2011 WRX would get at least the MPG that the sticker says 19/25 I would be happy with it and order one this weekend.
Sorry for the rant guys my first post and probably the one that will help me the most to spend my $ wisely on a new car.
Thanks a lot for any input
V
Thanks for the input 06scoobyrex. I have a really light foot so I guess that shouldn't be a problem. I see that User ID says 06 I was wondering if there are any 11 owners and if their average is anywhere near yours. Probably not, they are all excited and to happy with their brand new cars to spend any time on the these boards for a while :thumbup:
I got my 2011 in November, I've got a little over 3k on it now. My average for all 3000 miles is a little over 19mpg. Well over half of that has been highway driving. I tend to get about 17-18 city, and I don't really tend to dog it too hard on the regular. I took a rent trip to Austin (from New Orleans about 500 miles each way) and averaged just under 23 for the whole trip, from start to finish, including all of the city driving in between.

I've gotten up to 28mg on the highway before, but thats in low rpms of 5th gear, when you start to go over 70 you lose gas mileage pretty quick. Being that I averaged about over 70 coming from Austin, I think I could have probably gotten here with better mpg, but I was in a hurry.

Let us know how your decision
 
#168 ·
Hey V, assuming you don't drive WOT everywhere you can easily hit the projected mpg.

I took a trip up to Erie PA from Pittsburgh and was going 75 the whole way there and got just over 25mpg, slightly less on the way back with a full load in the car (Helping a friend move). as far as "city" driving goes. My commute to work is 4.5 miles and with letting my car heat up in the mornings before I go and the commute is all up and down hills with a lot of stop signs and stop lights, I'm sitting at about 17-18 mpg (i do open it up a few times on my short ride as well just to make it fun and it's twisty roads). If you baby the throttle and are on a highway you will get high 20's. Good luck in your decision. These cars are amazing.
 
#169 ·
Yeah no problem man... the 06's have a smaller TD04 turbo with the same engine and transmission and the 11's have a larger VF52 turbo... so I'd think you might loose about 1-2 miles per gallon, but then again your biggest factor is driving habit... these cars have very bi polar engines that have the ability to put out as much power as a v8, so gas mileage can be very different and there are a lot of different variables... good luck and post up some pictures when you do get your rex...
 
#171 ·
1,500 miles on my 2011 WRX. My work commute is around 25 miles and I'm averaging 22-ish mpg mostly city driving (60mph cruising though, not downtown stop-n-go). So that's kinda highway-ish driving but with lots of lights. I drive with a fairly heavy foot but when I really get on it a lot, it only drops 1-2 mpg.

I'm honestly amazed by how good it's been. My 22-ish average is both stock and at stage 2. I noticed Stage 2 drop it a bit but seriously, not much at all. All highway driving the car will see 25mpg no problem.

Coming from an Evo 8 that has the smallest gas tank in the history of all cars and gets MPGs in the mid teens...the WRX is performing phenomenally!
 
#173 ·
I'm over 4k now and my last receipt-vs-mileage test put me still in the 21.x mpg range. That is 50/50 city/hwy but the city stuff is usually 60mph long streets and then a series of short stop-n-go lights. So it's not like driving downtown. And I drive my car pretty hard for the most part. Plenty of boost and at least one really good pull through the gears a day.

I get 300 miles per tank no matter how I drive. To me that's great.
 
#175 ·
On my "old college daily commute" (70 mile round trip, 70% Highway w/ 55 mph speed limit and 30% City, altitude change from 9000 ft to 6000 ft to 9000 ft daily).

My college daily commute got me an average of 25-27 MPG and thats with occassional flooring the gas pedal.

My new work commute is around 35-40 miles round trip, roughly 90% highway driving w/ 65 MPH speed limit, minimal altitude change, Highway frequently subject to slow-downs. Got around 27 MPG Avg on stock tune. (also including a weekly 110 mile mountaind drive and 40 mile interstate drive w/ 75 mph speed limit).

Now on stage 1 tune and havent checked mpg yet. Also fumbling around economy tune as well. Will update my findings.
 
#180 ·
I'm making a drive up to San Fran (roughly 400 miles) in 2 weeks. I'm interested in seeing what kind of mileage I get doing all highway. If I'm in the 21.x mpg range now, I'm expecting 24mpg or better if I keep my foot from getting too heavy lol. :cool:
 
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