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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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#185 ·
K well kinda funny and by the way im canadian and i do metric soo kilometers. alright soo i picked up my accesport today for my 07 wrx and in the city on economy mode it stated i was getting 7-8 Liters on 100k. which is decent. got on the highway it got up to 9 and this is going 110kph. I change the map on the go to stage 1 and get her up to 160 and it read 33 liters for every 100k. My question is how much can an 07 wrx tank hold? andd damm is that ever bad
 
#186 ·
I got 25.3 MPG on my first tank in my 2011. That was calculating it based on miles driven/gallons added. I found the MPG gauge on the dash not accurate as it was reading 23.x MPG. My typical driving is country roads. I'm very satisfied with that compared to my Touareg that was averaging about 16.8 MPG...
 
#187 ·
On my new 2011 with 550 miles currently, I'm getting about 21-23mpg based on the trip computer (mixed city/hiwy - no cruise ctrl and 65-75 on the hiwy with the occasional foray into boost at green lights though I try to drive easy). The worst I got was 18 in stop and go traffic. I'm hoping it will get better as it gets broken in.
 
#188 ·
Had my WRX since mid Feb., bought new with 108 miles on it (some of the local dealers will drive new cars from dealership to dealership rather than flat-bedding them... boo! bad form! but I can't blame them in the case of a rex!), it now has about 3500. So far the only things I have done to it are 4 bolting the license plate with hex bolts and silencer washers, changed the oil once (Mobil 1 full syn 5w30, at 1500mi) and applied lots of wax to the paint. At first I was getting about 18 MPG average (75% city, lots of hills) according to the OBC. Recently average has increased to about 19.5 MPG (also on the OBC) with same driving habits and routes. My right foot is so heavy it has it's own gravitational force.:D

I red-line as often as I can, or at least get to about 4.5-5K RPM. There have been a few times on the highways that I have noticed on flat stretches holding a constant speed in the right gear my WRX will get upwards of 30MPG, but I shoot that number back down either accelerating or when the road starts going up hill again

As a comparison though, my last car was an 05 Scion TC (2.4L N/A I4, 161HP on the sticker, and the 4spd auto...bah!) and I drove it the same way (though not at as high of speeds) and averaged about 19 to 21 MPG in it.

DISCLAIMER: I neither advise or condone speeding :nono: .... As your father used to say, do as I say, not as I do:rotfl:
 
#189 ·
I personally find all this a total joke cause the government and news makes it a big hype. Everyone has a cost of living. filling your car or truck is one of them. I dont really think the price of gas has ever gone up or down becuase as we the people get paid more the cost of items slowly rise also. look way back when. My uncle had a challenger for 2600 back in the day and that was a lot and gas was like 30 cents now a challenger is like 30K from what i've found and gas is roughly 389 by me last i fulled up... I drive a 86 ford f350 with a 5.9 cummins swapped. with it Non I/C i was getting 19-21 no matter how i drove it. now i I/C'd it and i got 22.3 last time i filled up and that was half non I/c and then I/c'd.

My brother drive s a 05 Chevy Duratrash with 4 inch exhaust cold air intake bully dog six gun tuner i think ( know its a bully dog) and he got 26 mpg highway and the tuner off.

In my mind if you want MPG get a moped. in order to have power and go fast it gonna cost ya. by A) a REALLY good design that is cheap to run ( fuel) or B) a old school big block that just sucks the gas. not saying you shouldnt try and get better mpg but i think this stuff gets blown out of the water and makes it seem like the end of the world...
 
#191 ·
22.2 mpg (10.6 L/100km) recently, all city. The only thing I did differently was keep it at 3k rpm (minimum) when I could, so I had my power readily available and got decent mileage.

Monitor mileage as another means of gauging vehicle health.
 
#192 ·
Last Fill-up, roughly 80% Highway driving. Traveled 405 miles on 13.6 gallons. Averaged 29.78 MPG on Stage 1.

Not too shabby, Usually tried to baby the throttle and stay out of boost, Also went at or 5 mph below the speed limit.

Trying to see if I can break the 30 mpg avg.
 
#195 · (Edited)
That's great numbers for the 2.5!

Just filled up last night and got 29.7 mpg on my last tank with about 70% hwy/30% city. That was using the following "hypermiling" driving habits:
- Stay out of boost.
- Shift between 2,100-2,300 rpm going down a hill.
- Shift at 2,500 rpm driving on flat road.
- Shift at or below 3,000 rpm when going up a hill or as soon as the grade starts to lessen and shifting won't cause the turbo to start boosting.
- Compression brake whenever possible.
- Look as far down the road as possible to foresee lights changing or traffic slowing.
- Maintain the speed limit or a maximum of 5 mph over.
- Use cruise control whenever traffic conditions allow.

I think that's about the best I can do without taking it on a non-stop road trip. Driving in Seattle traffic is hard to keep good mileage. This is with a stock tune, K&N drop in filter and GrimmSpeed ceramic coated catless uppipe. Hopint to purchase a Cobb AP v2 before the end of the year and go to stage 1:).
 
#202 ·
Hell yeah, my MPG gauge says 76.6 (or something random like that) when I reset it going down hills and that is WITH the AC running! :D Now if only we had any idea of how to create that technology...

I've found something strange though. In a ~15 mile canyon between two valleys near where I live (going from Brigham City to Logan if you're familiar with the area) if I reset the mileage calculator in say ... Ogden... I'm usually getting about 27mpg (highway) when I get to Brigham. Then as I go up the canyon my MPG drops to about 23-24ish, but then if I put the car in neutral and let it roll out of the canyon into the other valley, putting it in gear in known speed trap zones of course, I typically arrive in Logan getting around 27.6-27.9 mpg. It works the same coming back the other direction too.

So the moral of the story is... if you travel up and down steep hills that will keep your cars speed up without any gas, if you simply let it roll you will at the very least match the average mpg you were getting before coming into the canyon over the entire length of the canyon.
 
#197 ·
Yeah me too. It's especially hard when people cut me off for not accelerating fast enough for them off a light. I guess the redeeming factor is when I pull up right behind them at the next light and wave:wave:, knowing their speeding up only to slow down only got them bad gas mileage. Ha ha.

Also the less wear and tear on everything is a big factor to me. The car has almost 149,000 miles and I can tell the trans is getting tired. About ready to start looking for a replacement unit or start saving for new gears and a rebuild.
 
#203 ·
If we could economically control gravity on localized basis, I suspect that cars (as we know them, anyway) would go the way of the horse and buggy.

 
#204 · (Edited)
Impressive Documentation

I think this documentation took a very long time to produce and it seems to contains valid experience from real drivers. Thanks for taking the time to maintain it.

I'd like to introduce myself. Call me Tommy. Other than knowing how to drive a car, I know nothing about cars. I figured this would be a good place to start learning.

I just purchased a 2012 Subaru WRX STi about two weeks ago. It is an amazing ride. My route to work and home is about ~100 miles a day. It's probably a little more than 800 miles on the odo now.

I've only been to the office 7 day these past two weeks, but I'm normally there 5 days a week. The images showed my current lifetime odo reading which is the same as the trip A reading and trip A's mpg reading.

Those readings are based on my driving style when I brought it at 10 miles...

98% Highway / 2% City - 8 out of 14 days
65% City / 35% Highway - 6 out of 14 days

Avg. % of Shifting less than 1.8k rpm - 2%
Avg. % of Shifting around 2.3k rpm - 18%
Avg. % of Shifting around 2.8k rpm - 20%
Avg. % of Shifting around 3.2k rpm - 50%
Avg. % of Shifting around 3.5k rpm - 6%
Avg .% of Shifting around 4.0k rpm - 3%
Avg .% of Shifting greater than 5.0k rpm - 1%

97% Intelligent mode
2% Sport Sharp / S# mode
1% Sport / S mode

Highway Cruising in 6th at around 2300rpm [50%] - 3100rpm [50%]
I have the rev set at 3200rpm which keeps me under 80mph.
I like to boost on the way to work. (5-10 second boosts, 2-3times a trip. 10 second+ boosts, 1-2 times a trip, 2-5 minute boosts, 1-2 times a trip)
I stay off the boost on the way home with minor exceptions. I usually boost uphill when the lane is clear.
I stay off the boost at least two miles before I reach my destination, and I leave the car idle for two or three minutes when I get to the destination.

It probably did 80mph for ~3 hours during it's life so far.
It probably did 90mph for ~15 minutes during it's life so far.
It probably did 90mph+ for ~3 minutes during it's life so far.
It probably did under 80mph for the rest during it's life so far.
 

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#234 ·
I think this documentation took a very long time to produce and it seems to contains valid experience from real drivers. Thanks for taking the time to maintain it.

I'd like to introduce myself. Call me Tommy. Other than knowing how to drive a car, I know nothing about cars. I figured this would be a good place to start learning.

I just purchased a 2012 Subaru WRX STi about two weeks ago. It is an amazing ride. My route to work and home is about ~100 miles a day. It's probably a little more than 800 miles on the odo now.

I've only been to the office 7 day these past two weeks, but I'm normally there 5 days a week. The images showed my current lifetime odo reading which is the same as the trip A reading and trip A's mpg reading.

Those readings are based on my driving style when I brought it at 10 miles...

98% Highway / 2% City - 8 out of 14 days
65% City / 35% Highway - 6 out of 14 days

Avg. % of Shifting less than 1.8k rpm - 2%
Avg. % of Shifting around 2.3k rpm - 18%
Avg. % of Shifting around 2.8k rpm - 20%
Avg. % of Shifting around 3.2k rpm - 50%
Avg. % of Shifting around 3.5k rpm - 6%
Avg .% of Shifting around 4.0k rpm - 3%
Avg .% of Shifting greater than 5.0k rpm - 1%

97% Intelligent mode
2% Sport Sharp / S# mode
1% Sport / S mode

Highway Cruising in 6th at around 2300rpm [50%] - 3100rpm [50%]
I have the rev set at 3200rpm which keeps me under 80mph.
I like to boost on the way to work. (5-10 second boosts, 2-3times a trip. 10 second+ boosts, 1-2 times a trip, 2-5 minute boosts, 1-2 times a trip)
I stay off the boost on the way home with minor exceptions. I usually boost uphill when the lane is clear.
I stay off the boost at least two miles before I reach my destination, and I leave the car idle for two or three minutes when I get to the destination.

It probably did 80mph for ~3 hours during it's life so far.
It probably did 90mph for ~15 minutes during it's life so far.
It probably did 90mph+ for ~3 minutes during it's life so far.
It probably did under 80mph for the rest during it's life so far.
Welcome to the club and thanks for posting up your information :)
 
#206 ·
Hey
somewhat relevent, I can't get my miles per gallon thingy to appear. A disappeared when the car went to the dealer and I can't it back on. The manual said to put key in the lock position and press the plus button, the thing blinks but the miles per gallon still doesn't show up.... any help?
When you put in the key, the key is in a lock position right?
 
#210 ·
I checked mine today when I filled up and I got 24.3 MPG so this next tank I go through I'm going to do some really easy driving, shifting 2,500 RPM and lower and try my best to time stop lights and run a few stop signs along the way lol
 
#211 ·
That's pretty good for an STi. I filled up yesterday in my stage 1 '02 WRX and was at 27.67mpg. I generally keep it below 2,750 when accelerating and in-gear coast. I found that cruising in 5th at 30 mph nets good results too:).
 
#213 ·
I just filled up my tank while I was sitting on roughly half a tank according to my guage. It only took 5.5 gallons my trip odometer read 187 miles. That's exactly 34mpg. I don't know if I didn't squeeze enough gas in my tank or what. I let the pump shut off, pulled the nozzel out and after a little more it clicked off again. Not sure if I'm retarded or if my new car is super car in town here. It was all town, no highway driving, but not a whole lot of stop and go city traffic.

2004 wrx stage 2 tune from cobb ap, turbo back exhaust catless, and an intake.

I haven't owned the car long, this is the first tank I haven't really drove the car hard. Earlier tanks were about 25mpg though. A full tank of gas is about 12.5 gallons for my car.
 
#216 ·
Here is my recorded data for the first 55k on my 2011 wrx wagon. I drive it pretty hard and live at 7k altitude. I use the max octane which is 91.
edit: fyi, this value is about 1-2 mpg less than the digital readout.
View attachment 104367
Nice data logging! I'm surprised you do as well as that at high altitude and with 91 octane
 
#217 ·
Did some real easy driving since the last fill up and got 25.4 MPG. That was with shifting at no higher then 2,500 and shifting below that as much as possible and trying to time the lights the best I could, worked sometimes but not always.

I thought of another idea as well. A lot of the time I can tell I have to stop and I am carrying a lot of speed like an up coming light or backed up traffic. My next test tank I go through whenever possible I am going to take it out of gear and cost as much as I can w/ out impeding the flow of traffic. If I can shift it into neutral and coast using no RPM's that should make a difference from getting out of the gas but still turning 2,000 RPM's. Its the RPM's that really knock down the gas mileage and sitting in one spot with out moving.
 
#222 ·
If I can shift it into neutral and coast using no RPM's that should make a difference from getting out of the gas but still turning 2,000 RPM's. Its the RPM's that really knock down the gas mileage
When you coast while in gear using engine braking you don't use any fuel as the injectors are shut off and the motor is kept turning by the momentum of the car. Since there is no combustion going on you get engine braking since the friction and force required to pump air in and out of the cylinders work to slow the pistons down. Coasting in neutral, the car needs to use gas to keep the motor turning instead of using the wheels to turn it so in theory you should use more gas.
 
#220 ·
100% bone stock, '03 5-speed Bugeye wagon, 87k on the mill.

Some days I shift at 2500, some days a shift at 6500. There's not much rhyme or reason. I just drive, I enjoy it.

I'm getting 24.1 MPG. I switched from Sheetz 92 octane gasoline to Sunoco 93 tonight when I filled up. It really seemed to make a difference on the 11 mile trek home. It could be all in my head, though. :)
 
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