drewkitty ([info]drewkitty) wrote in [info]poor_skills,
@ 2007-12-29 14:52:00
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surprising tips for good gas mileage
A prior poor skills post about long range commuting caused me to go out and gather a bunch of tips for improving gas mileage.

Driving Habits and Accessory Usage

  1. reduce your speed! biggest saver (every 5 mph over limit equals 7% decrease)
  2. on high speed freeways (65 and 70 mph) drive the speed limit
  3. combine errands to make a "great circle" rather than several "to and from" trips
  4. warming the engine uses fuel, so combining trips while the engine is warm saves gas
  5. accelerate smoothly and evenly, avoiding "mashing the pedal"
  6. try to avoid driving faster than necessary into a head wind (you can feel the buffeting)
  7. decelerate only as needed, avoiding hard braking
  8. idling gets 0 mpg! so avoid it (i.e. drive-throughs, waiting for kids at school)
  9. know your roads and routes and avoid clogged routes when possible
  10. know your traffic light timings, driving the speed limit will often give you mostly green lights
  11. know your vehicle's transmission and engine "sweet spots" and use the highest gear practical i.e. overdrive gear on the freeway
  12. use cruise control on long-distance flat roads, but don't use cruise control on uneven terrain (hills)
  13. park in the shade during the day to avoid fuel evaporation
  14. for cooling, use the ac at highway speeds; use the window for city driving



Maintenance

  1. remove excess weight from vehicle (save 1% to 2%)
  2. only mount a roof rack or carrier when needed, extra weight and aerodynamic drag
  3. clean and wax your car, especially before a long trip (disputed, <1% to 7% savings)
  4. inflate your tires properly, check monthly (find proper PSI not on sidewall which is 'max' but on doorjamb sticker or owner's manual) savings 3% or more
  5. clean air filter according to owner's manual (often popping off a nut and vacuuming!)
  6. replace air filter (10% savings or more)
  7. replace clogged fuel filter
  8. check the oil and have oil changes done as needed
  9. have regularly scheduled tune-ups performed (to check fouled spark plugs, oxygen sensor)


Purchases

  1. gasoline: use the correct gasoline for your engine, per manufacturer spec -- often plain jane unleaded
  2. gasoline: purchase during the cooler hours of day or evening, as heat makes gas expand and you get less energy per 'gallon' during middle-of-day purchases
  3. gasoline: avoid over-filling, once it 'clicks' it's usually enough
  4. gasoline cap: don't forget to replace it! otherwise you are letting gasoline vaporize out of the tank
  5. motor oil: use recommended grade, again per manufacturer spec
  6. tires: use radials, get lighter rims ("unsprung weight")


Sources Included:

- http://www.wanderings.net/notebook/Main/SurprisingWaysGetBetterGasMileage
- http://www.stretcher.com/index.cfm
- http://www.edmunds.com/reviews/list/top10/103164/article.html
- http://www.leemyles.com/articles/clean-car-for-good-gas-mileage.html
- http://www.thesimpledollar.com/2007/09/02/the-one-hour-project-basic-car-maintenance/
- http://www.gaspricewatch.com/new/f-resources.asp

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[info]brownizs
2007-12-29 11:25 pm UTC (link)
Number 2 under Purchases is actually Fallacy, due to time of day does not matter. Pumps are calibrated by Weights & Measure for the state, and liquid content does not change, no matter what the temp or ambient temp outside.

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WRONG
[info]drewkitty
2007-12-29 11:41 pm UTC (link)
http://www.poynter.org/column.asp?id=2&aid=134222

This sounded crazy to me. I mean, the tanks are underground -- how much heat or cold could actually get into them? But in 2006, The Kansas City (Mo.) Star investigated the story and found it to be rooted in truth. Temperature really does have something to do with gasoline density.

In fact, it could amount to a few pennies per gallon that motorists get shorted in the summer. There are some who argue that pumps should be temperature-adjusted as they are in Hawaii. NPR picked up the story this summer.

http://abcnews.go.com/GMA/Consumer/Story?id=3022167&page=1

Are You Getting Ripped Off by 'Hot Gas'?

Nine Lawsuits Claim Gas Stations in U.S. Should Control for Outside Temperature to Make Sure Fuel Doesn't Expand

There are at least nine lawsuits pending that claim some gas stations are padding their profits by selling warm gasoline.

That might sound strange, but think back to high school physics. Liquids expand as they get warmer, so if the gasoline at the pump is overheated, you don't get as much in your tank.

. . . When the temperature outside gets above 60, gasoline at the pump expands, and you get less of it.

Physics professor Michio Kaku demonstrated the concept by heating water in a bottle. As it heated up, it expanded and overflowed. In other words, it took less water to fill the same size container. "Remember that gasoline expands roughly three times faster than water," Kaku said. "So magnify this effect inside your gas tank."

http://www.consumerwatchdog.org/energy/nw/?postId=6759
reprinted from The Kansas City Star, August 27, 2006
by Steve Everly, The Kansas City Star

Hot fuel for you and cold cash for big oil:

When gasoline gets hot, it expands. But U.S. fuel pumps don't account for the bigger volume, and it's costing American consumers about $2.3 billion a year.

. . . As a liquid, gasoline expands and contracts depending on temperature. At the 60-degree standard, the 231-cubic-inch American gallon puts out a certain amount of energy. But that same amount of gas expands to more than 235 cubic inches at 90 degrees, even though consumers still only get 231 cubic inches at the pump.

Put simply, every degree over the 60-degree standard diminishes the energy a 231-cubic-inch gallon delivers to the nation's fleet of cars, trucks, boats, buses and heavy equipment -- and forces drivers to consume more and pay more for fuel.

It is basic physics that, depending on the temperature, can amount to just a few cents per gallon. But it adds up to big money -- coming straight out of consumers' pockets and going right to the bottom line of major oil companies and other fuel retailers in the energy pipeline. Moreover, it's perfectly legal, because even though your local filling station measures out your gas as if it were stored at 60 degrees, no law requires retailers to adjust the pump to reflect the expansion of hot fuel.

. . .Industry and government officials have known about the hot-fuel issue for decades. The 60-degree standard was set by mutual agreement of the oil industry and weights and measures regulators roughly a century ago.

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[info]drewkitty
2008-01-06 03:15 am UTC (link)
Other advice from a fuel expert I spoke to included:

- fill tanks when half full, so that excess vapor does not go back out of tank
- do not fill if you see a gasoline tanker refueling the station, as this kicks up sediment that may end up in your gas tank
- do not fill past the first stop point, as you are merely kicking vapor back into the station's systems
- fill at the 'medium' speed -- filling too fast also kicks up excess vapor for the return system to enjoy

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[info]brownizs
2008-01-06 03:56 am UTC (link)
Again, all misinformation. Everything you posted is not true.

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Important Disclaimer
[info]drewkitty
2008-01-06 11:33 am UTC (link)
Some more detailed research revealed that these four tips are based on an E-mail meme reported to snopes.com in February 2007. Your mileage may vary.

http://www.snopes.com/inboxer/household/gastips.asp

Note however that snopes.com lists the meme as "indeterminate."

The short form answer is that half fill vs. quarter fill is disputed; sediment kick-up is unknown (but what does it hurt to pass up a station during its refueling?); topping off is definitely bad and a waste of money; and the speed of filling should not affect the vapor.

- (1) Fill when half full

http://wiki.answers.com/Q/Is_it_better_to_fill_up_a_gas_tank_when_it_reaches_half_or_almost_empty
(indeterminate)

http://www.blogher.com/3-new-gas-saving-tips-thanksgiving-when-every-penny-counts
(commenter recommends minimizing number of refuelings to avoid vapor loss)

http://www.cartalk.com/content/columns/Archive/1994/March/10.html
(neither condensation nor sediment are good reasons to fill at half full)

The argument seems to revolve around whether loss of vapor by refilling at 1/4 tank is more than the extra weight of carrying around fuel between 1/2 and 1/4 tank. General agreement that filling at 1/4 tank is prudent.

- (2) Tanker refill / sediment

http://answers.yahoo.com/question/index?qid=20071117080316AANqY9E
sediment suspended in fuel during refueling process

http://mythbusters-wiki.discovery.com/thread/1105897/useing+gas+station+while+tanker+is+there?offset=0&t=anon
disputed, many variables

Point is made that gas stations have filters and most change them regularly. Some do not and particular stations may give consumers problems in this regard. No solid information either for or against.

- (3) Don't fill past first "stop" point

http://answers.yahoo.com/question/index?qid=20070815163128AAEKdPm
"topping off" spills vapors, temperature expansion, affects both car and station's vapor recovery system

http://www.physorg.com/news7468.html
"topping off" puts fuel into car's vapor recovery system

http://www.gassavers.org/showthread.php?p=87427
"spilling gas & fouling recovery system"

http://www.goodwrench.com/Tips/FuelEfficiency.jsp

http://www.husky.com/apetnoz.htm
discussion of nozzle tips, 'peening' and how shut off system works

http://www.des.state.nh.us/factsheets/rem/rem-25.htm
schematic of gas station's vapor return system
"Stage II Vapor Recovery System"

http://www.familycar.com/Classroom/emission.htm#EVAPORATIVE%20CONTROLS
description of car's evaporation control system

Confirmed: topping off is bad for the environment and dumps fuel you paid for, either into the air, onto the ground or (in some states) into the gas station's vapor return system. Also not good for your car's vapor return system.

- (4) Fill at "medium" speed

No evidence in favor of this could be found.

http://www.nbc4.com/consumer/9842495/detail.html
testing protocol in VA is to fill twice, once at slow and once at fast

In theory, slow speed should reduce vapor -- but gives more time for vapor return system to displace. Fast speed would kick up more vapor, but total exposure time would be less.

- Multiple comments

http://www.rgxlife.com/freshink/articles/gas_tips.html
agrees with 1) 2) 3) 4)

http://seattle.craigslist.org/see/rnr/527822979.html
agrees with 2) and disputes 1) 3) and 4)

- Interesting

http://www.sfgate.com/cgi-bin/article.cgi?file=/c/a/2007/01/28/CMGR8ND0EB1.DTL
interview with a gas station owner

http://boards.fool.com/Message.asp?mid=26040462&sort=whole&source=ihtfoceml765000
long argument about temperature compensation, underground tanks, and change of temperature between tank storage and dispensing

(Reply to this) (Parent)

[info]chaeri
2007-12-29 11:53 pm UTC (link)
actually, you can top up the gas tank. many gas station nozzles stop a good deal short of where you can actually fill it.

as far as speed limit, it depends on how long it adds to the drive to go the speed limit vs what you drive. the best speed is 55mph but of course i can't do that on the 65mph roads - i'd be hit. if i went the speed limit on the 25/35mph road it would take me an extra 20 mins to get anywhere. i don't think going 40mph is going to burn enough gas to equal that extra 20 minutes or so.

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[info]gentlemaitresse
2007-12-30 01:00 am UTC (link)
It doesn't say you can't, it says there is no need. And I've read elsewhere that's it's bad to do so, though I can't remember why, exactly. It probably has something to do with allowing room in your tank for expansion.

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[info]chaeri
2007-12-30 01:55 am UTC (link)
i'm saying thats false, or depends on the make and model of the car.

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[info]robinhoodvandal
2007-12-30 02:02 am UTC (link)
I think the best reason to stop when the pump does is that it minimizes the chance of spilling gas. You don't want to pay for gas on the ground.

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[info]gentlemaitresse
2007-12-30 02:06 am UTC (link)
You can ruin the carbon filter, and possibly the gas sensor, by overfilling. There's no reason not to stop when the pump shuts off automatically. Why take the risk?

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[info]drewkitty
2007-12-30 01:25 am UTC (link)
For most cars, the sweet spot is between 45 and 50 MPH. This is where the engine is most efficient at producing work, and the transmission most efficient at putting that work to the road.

>> on high speed freeways (65 and 70 mph) drive the speed limit


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[info]chaeri
2007-12-30 01:54 am UTC (link)
meh, i've always heard between 55-60 - from every mechanic i trust plus anyone who knows anything about cars. might depend on make and model.

well, honestly, can't go the speed limit safely on the highway either. i'd get rearended.

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[info]drewkitty
2007-12-30 08:25 am UTC (link)

Note the chart here: http://eartheasy.com/live_fuel_efficient_driving.htm

This shows the PEAK at 55 mph and steadily dropping to 60 MPH.

Engineering details here: http://auto.howstuffworks.com/question477.htm

"Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds."

Also http://answers.google.com/answers/threadview?id=21

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[info]pixi_stardust
2008-02-07 05:56 am UTC (link)
where do you drive where people are going to hit you because you're going the speed limit?

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[info]chaeri
2008-02-07 12:36 pm UTC (link)
its been that way on any highway i've been on, especially if you are in the left lane. we also have ridiculously low speed limits - 55-65 for highway driving. i live in MD which might explain it.

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[info]pixi_stardust
2008-02-08 06:12 am UTC (link)
well the left lane is meant for passing people, and the right lane is meant for the slower traffic, so if you're going the speed limit in the passing (fast) lane. many highways even have signs that say "slower traffic keep right"

i've driven in kansas city, denver, chicago, st louis and san francisco and i've never really had a huge problem with going the speed limit in the right lane.

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[info]chaeri
2008-02-08 12:35 pm UTC (link)
ah, see we don't enforce traveling in the left lane as a ticket or anything. TECHNICALLY yes, its just for passing and turning but rules are only as good as they are enforced. we don't have any such signs. right, you might have been fine on those roads. i've just had a difference experience. MD is crazy.

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[info]thisisallyou
2008-02-29 08:21 pm UTC (link)
if you're driving slower than the flow of traffic, you should not be in the left lane.

You should be in the furthest right lane that is going at the speed you want to go to.

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[info]gentlemaitresse
2007-12-30 02:02 am UTC (link)
As for saving time, in order to save 20 minutes by going 40mph instead of 35 you'd have to go a long distance. Each hour you're only saving about a minute, or just a little more. Do the math.

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[info]chaeri
2007-12-30 03:24 pm UTC (link)
its 2 roads, 25-35 each, i go 40-50 straight down (unless i get stopped by a train). i've watched the clock.

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[info]trip_tych
2007-12-30 01:07 am UTC (link)
"reduce your speed! biggest saver (every 5 mph over limit equals 7% decrease)"

How does that work? It seems kind of relative...if you are in a 35mph zone and go to a 30mph one and don't drop down the 5, how does it suddenly use more gas?

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[info]drewkitty
2007-12-30 01:20 am UTC (link)
National speed limit of 55 mph. Good catch.

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[info]daniilm
2007-12-30 01:17 am UTC (link)
Driving at speeds that are less than that of majority of other traffic compromises your (and others) safety. Not sure I'd trade higher MPG for safety, but for those who would I can add:

1) Inflating tires few PSI above what it says on the sticker is generally safe enough and reduces rolling resistance thus saving MPG, but might reduce grip in some situations.
2) Following 18-wheelers just a few feet behind on the interstate is also not too safe, but does save mpg.

Safer advice:

1) Using lighter oil weight in winter (that is still manufacturer recommended) will save some mpg (might leak more though).
2) Do not go for the cheapest gas, buy cheapest among decent places like BP, Shell, Mobil. They typically have better quality gas, and add injector/fuel system cleaning additives that save MPG.
Or at least buy some fuel system cleaners in walmart once every few thousand miles.

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[info]drewkitty
2007-12-30 01:23 am UTC (link)
I DO NOT recommend drafting behind trucks. It severely aggravates the truck driver if they saw you (as now they have to drive like they're on eggshells), and severely endangers you if they did not (as if they suddenly hit the brakes to react to road hazard, you will become one with the trailer rear bumper.)

No amount of fuel savings can buy you a set of new air bags, let alone a new face. Reasonable driving speed must be balanced against safety. Generally "with the flow of traffic" and following the 2 second rule. If you're going to cruise for fuel economy, do not do it in what some states still call the 'passing lane' or leftmost lane.

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[info]robinhoodvandal
2007-12-30 02:04 am UTC (link)
overfilling your tires generally reduces the life of the tire. I don't know if that would be made up for by the fuel saving though...

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[info]daniilm
2007-12-30 02:49 am UTC (link)
If you overfill them by like 10+ psi, they might wear out in the middle faster, however 2-3-4 psi will not affect tire wear much...

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[info]macaroniandtuna
2007-12-30 03:39 pm UTC (link)
DO NOT draft. Plain and simple. You have to be very, very close (about 2 feet behind the truck/car/whatever you're drafting) to get any noticeable benefit, and if they even lightly brake, you'll rear-end them. It's not worth it.

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[info]daniilm
2007-12-30 04:43 pm UTC (link)
actually no, even 100 feet behind saves like 10%, and 50 feet (iirc) saves like 25%. Mythbusters did an episode on this.

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[info]macaroniandtuna
2007-12-30 05:05 pm UTC (link)
Fair enough, but at highway speeds even 100 and 50 feet distances are very dangerous. 60mph equals 88 feet per second, so at 100 feet you've got barely more than a second to react and at 50ft less than a second. Most people travel faster than 60mph (65mph=~95fps, 70mph=102.667fps, both of which are common interstate speed limits), cars therefore travel farther in the same amount of time, decreasing reaction time. It also takes roughly half a second for anybody to react to anything at all (roughly 300ms for signals to be received and interpreted by the brain, then another 200ms for a response to be sent to the muscles), increasing the danger even more.

Long story short, the increased fuel mileage from drafting isn't worth the danger and increased crash risk, period.

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[info]daniilm
2007-12-30 06:19 pm UTC (link)
Yes, but how many people keep 2 second (150+ feet) distance in regular driving? Driving less than 2seconds behind big SUV, consumer truck or tinted large sedan is dangerous too, but every day I see many people doing it. Aparently they are fine with increased risk, so they should be fine with increased risk associated with drafting.

As for dangers of drafting, the major one is not the danger of not having enough time to brake and hitting the truck. Trucks cannot (or rather almost always do not) brake as hard as passenger cars, so less than 2 seconds is not the major threat. What is dangerous - if the truck is blocking the view of the highway curve with no curb (or snowy/dirty one) or with some construction blocks etc.

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[info]macaroniandtuna
2007-12-30 03:52 pm UTC (link)
A couple things:

-As others have pointed out, that 5mph over=7% reduction in mileage is relative. A better way to point that out is that wind resistance (= fuel consumption, roughly) increases with the cube (x^3) of the speed. I don't know the exact equation, but that's a pretty convincing argument to me anyway.

-Unnecessary deceleration wastes fuel because you then have to use more to get back to speed, yes. But braking does not in itself increase fuel consumption, no matter how hard you brake. I've never understood why all these lists say "brake gently." It might save wear and tear on brake pads/discs, but it won't save fuel.

-11 could be a bit clearer, methinks. I can put my car in fifth gear at 30mph, but that would use a ton of fuel as the engine's rpms are so low that you have to lug the engine hard to make it go. Gasoline-powered cars rarely put out enough power below ~2000rpm to be of much use.

-13 - If your gas tank is closed, fuel evaporation shouldn't be a problem, as the evaporated gas has nowhere to go. This tip doesn't make much sense from a fuel-saving standpoint.

-A/C compressors are run on a pulley system like the other accessories, so they take a percentage of the engine's power. It's actually more efficient to cool yourself with the windows down, even at highway speeds, than it is to use A/C (says Mythbusters). There is of course the comfort factor, though, as A/C is much quieter at highway speeds than the wind and of course cooler. So it's a tradeoff.

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[info]daniilm
2007-12-30 05:02 pm UTC (link)
Wind resistance is actually quadratic in speed, not cubic.
Most efficient way to brake - let go of gas so that car computer cuts fuel supply to the engine. Works best in a stick-shift, since you you can gradually put it in a lower gear. Not that efficient/does not work in some auto transmissions.

Opening windows at high speed increases parasitic drag of a car, hence costs extra fuel to maintain the speed. Actual comparision of windows down vs AC at high speed depends a lot on the car design. In an older truck windows down will probably not have as much of an effect on drag coefficient as in a newer smoother-curved (as a result of being tested in a wind-tunnel at development stage) sedan.

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[info]macaroniandtuna
2007-12-30 05:15 pm UTC (link)
1) Either way, an exponential increase is significant.
2) Does anybody *not* let go of the gas when they brake? To do so seems stupidly counterproductive, in addition to requiring that you brake with your left foot, which is bad technique (short of complex racing skills, which aren't all that practical for everyday use anyway).

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[info]daniilm
2007-12-30 06:29 pm UTC (link)
1) Erm, sure exponential increase would be significan, what does it have to do with speed^2?
2) What I mean is brake with not use of brake pedal at all until the last few feet before coming to a complete stop. One ends up with the same speed in the end, and does not use gas all the way up until that point, whereas braking in any way reduces speed faster = start braking later = extra distance to cover using gas.

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[info]macaroniandtuna
2007-12-30 06:49 pm UTC (link)
1) I don't understand your question.
2) Ahh, so you basically mean coasting. It's a good idea, I agree, but it is technically illegal. ::shrug:: Why it's illegal, I don't know. Probably a safety thing.

I think you're referring to engine braking rather than coasting, on second thought. Engine braking works just fine, but it causes extra wear and tear on the engine.

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[info]daniilm
2007-12-30 07:26 pm UTC (link)
1) Drag is proportional to speed^2, exponential=e^speed => has nothing to do with drag.
2) Coasting does not save gas, and yes I am talking engine braking. What kind of extra wear and tear on the engine from engine bracking are we talking about?

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[info]inquisitiveravn
2007-12-31 03:08 am UTC (link)
Actually, I think that engine braking increases wear and tear on the engine if you downshift to increase engine resistance. OTOH, I'll take wear and tear on the engine if it lets me keep control of the car in icy conditions which is about the only time I would downshift as a method of slowing down, or at least not accelerating excessively.

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[info]macaroniandtuna
2007-12-31 04:05 am UTC (link)
1) That x^2 has an exponent (2) makes it exponential. e^x is also exponential, yes.
2) Wear and tear that will reduce the total life of the engine and generally cause premature wear, ie oil leaks, increased fuel burning, increased emissions, oil burning, etc. etc. Whether it's enough to make engine braking eventually cost more than the fuel it saves, I don't know.

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[info]daniilm
2007-12-31 05:04 am UTC (link)
1) x^2 is a power function, not an exponent.
2) Specifically, what will wear and tear more than usual if I use engine braking (and switch to lower gear occasionally, but not into high rpms region)?

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[info]macaroniandtuna
2007-12-31 02:34 pm UTC (link)
The engine's internals, like the block, cylinder heads, cylinder walls, valves, etc.

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[info]daniilm
2007-12-31 04:32 pm UTC (link)
And why would all that wear more than usual, given that RPMs are normal, and there's not stress of burning fuel in the cylinders?

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[info]macaroniandtuna
2007-12-31 07:09 pm UTC (link)
I don't know, and I can't find any authoritative source on the subject. I've always been told that engine braking in cars (as opposed to trucks, which have a Jake brake that's specifically designed for engine braking) causes extra wear and tear.

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[info]katexas
2008-01-29 12:53 am UTC (link)
I've heard a few different lengths of time, but when I was living in Toronto I most often heard "10 seconds" as the rule of thumb for how long you can idle before it becomes less efficient than turning off your car and starting it again. (of course this is not recommended at traffic signals)

In that vein, they are talking about getting rid of Drive-Thrus. So if you're given the option, it's apparently "cheaper" to go in and order at the counter (less gas wasted).

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[info]avatar_sitar
2008-02-01 07:27 pm UTC (link)
warming the engine uses fuel, so combining trips while the engine is warm saves gas

That might work in most cars, but if you have a volkswagen forget it. Immediately.
You'll have to pay for it later down the road, in engine expense and because as a rule, volkswagens (especially bugs, the older and newer models) need to be warmed up, because they operate on a different radiator system (the older models of bugs are essentially modified airplane engines, thus they need to be warmed up in order to operate at a normal and efficient speed. They don't have water radiators, which in the winter months can be somewhat of a curse... but the gas milage is worth it.).

Also, if you have an older model of a vw bug, you can coast very easily down large hills in order to save petrol (gasoline). Though, I only suggest this to those that are either very skilled with such a thing, or you are quite sure you won't be caught, since it is quasi-illegal in some of the states.
Hope this helps!

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