Wikipedia:Reference desk/Archives/Miscellaneous/2013 July 11

= July 11 =

Most fuel-efficient way to drive a car with Manual transmission?
I heard from various sources that the most fuel-efficient way to drive a manual transmission is to "pedal to the floor" while short-shifting. This means to floor the gas pedal and then shift early to keep the RPMs low. Their rationale is that engines are most efficient when the intake manifolds are fully open (this occurs when you floor the gas pedal), but instead of letting the RPMs rise to a high number (and hence using a lot of fuel), you shift early to keeps the RPMs low. Is this true?

I also heard conflicting information that modern 4-cylinder engines are most efficient at high RPMs. Is this true?

Thanks. Acceptable (talk) 02:43, 11 July 2013 (UTC)


 * Start here: Power_band. 196.214.78.114 (talk) 08:29, 11 July 2013 (UTC)


 * "To lower your fuel costs, protect your vehicle's transmission and reduce emissions, change gears as early as possible and into the highest appropriate gear without "revving" the engine." Greener Driving
 * "...don't labour the engine but try changing up at an engine speed of around 2,000 rpm in a diesel car or around 2,500 rpm in a petrol car." Automobile Association - Eco-driving advice
 * "The higher the gear you drive in the lower your engine speed is, which can improve fuel efficiency. So change up a gear whenever you can, without labouring the engine." Fuel Academy
 * Finally Wikipedia has an article about everything - see Energy-efficient driving. Alansplodge (talk) 09:21, 11 July 2013 (UTC)


 * The answer is to do what an automatic gearbox does - which is to try to keep the RPMs around 2,500 regardless of speed. That's not always possible - and if you do it, you'll be doing a LOT of shifting!  As we discussed with you yesterday, habitually having the RPM's be much lower than that will "lug" the engine and wreck it.  Pushing the RPM's much higher than that is wasting fuel.  One of the reason that true hybrid cars like the Prius are so efficient is that they can run the engine at the perfect RPM no matter how the driver is behaving and let the batteries provide the ability for high horsepower situations where more RPM is needed. SteveBaker (talk) 12:59, 11 July 2013 (UTC)

Thanks, but while accelerating, is it more fuel efficient to floor the gas pedal and hence reach my target cruising speed faster, or is it more efficient to accelerate gently - burning less fuel than "pedal to the floor" but for a longer period of time? After you integrate these two scenarios, which one will be more efficient? Acceptable (talk) 05:11, 12 July 2013 (UTC)


 * Gentle acceleration is more fuel-efficient.   D b f i r s   15:41, 12 July 2013 (UTC)


 * Why? Gentle acceleration uses less fuel per unit time than hard acceleration, but you need to gently accelerate for a longer period of time to reach your target cruising speed. Acceptable (talk) 21:08, 12 July 2013 (UTC)


 * Harsh acceleration uses the fuel inefficiently. I recall once trying to accelerate away from the lights ahead of a jag that was revving it's engine in the next lane, and my instantaneous mpg display read 8 mpg.  I decided I wasn't wealthy enough to try that again!    D b f i r s   07:09, 13 July 2013 (UTC)


 * And in addition to the engine itself working inefficiently, hard acceleration will cause (greater) wheel slip, which in addition to being inefficient in terms of fuel consumption, will also wear the tyres quicker, resulting in your having to buy replacements earlier.
 * On top of all this, hardest acceleration at low speeds in low gear outside of a race track could cause alarm and danger to (not to mention earning the derision of) other road users, and even your losing control of the car.
 * As a (UK) gearstick driver of around 30 years' experience, about the only time I would floor the pedal would be on the joining slip road of a motorway (freeway) where I had for some reason been unable to accelerate to the prevailing traffic speed (typically 60–80mph) more gently. {The poster formerly known as 87.81 230.195} 90.213.246.168 (talk) 18:41, 13 July 2013 (UTC)


 * ... also I forgot to mention that what counts for efficiency is fuel usage per unit distance, not fuel usage per unit time.   D b f i r s   21:35, 13 July 2013 (UTC)


 * There seem to be two schools of thought, I see on the one hand the official organisations giving the advice to accelerate calmly.. Then there are the extreme tuners who want to break mpg records. And in that group you have the ones with hybrids, and those driving "old" classic cars,. I'm most familiar with the older cars, so here's the advice they give:
 * Ideally you should use the Brake specific fuel consumption map for the car/motor in question to determine the optimal way to drive. You find a whole collection of such maps here.
 * Car engines have an optimal working point where the energy delivered to the wheels (torque X rpm) per unit of fuel reaches a maximum (the lines with the lowest BSFC values in g/KWh) . This is usually at close to peak torque and at low rpm, say for example 2000. When you're accelerating, you want to stay in that area, almost(?) full throttle to get the high torque, and switching early to stay in the optimal rpm zone. That way you reach your cruising speed quickly and efficiently. (Some people will skip second gear)
 * The low rpm makes the difference between efficient driving and burning rubber and money, especially with a gasoline car where efficiency goes down real fast at higher rpm, due to combustion, air intake and exhaust issues ...
 * Slowly accelerating is just as bad, you're running at low torque, a point at which your engine is less efficient, and it takes much longer.
 * When cruising at low torque, the most efficient rpm will be lower still, perhaps 1300. the optimal point can't be deduced from the BSFC map, because these are based on full throttle measurements. You don't wanna go too low or the engine can't develop enough power anymore (lugging)
 * If you want to save more fuel you could try "sprint and coast": instead of cruising at say 30% torque 100% of the time, you do 90% torque till you drive 70-75, then coast in neutral down to 60, and repeat. That way your engine runs at the most efficient point, and while coasting you don't have transmission losses. You never coast when you have to slow down or stop btw, breaking on the engine (car in gear and foot off the accelerator) shuts of the fuel supply completely, in contrast to coasting where the engine is idling. Sprint and coast would seem the most inefficient way to drive if you read other sources. Perhaps it's more a road safety issue?
 * Can't guarantee the accuracy, your best option might be a forum where your car is discussed. Ssscienccce (talk) 22:28, 14 July 2013 (UTC)


 * The trouble is that:


 * Yes, you use more fuel in harsh acceleration.
 * But...the sooner you can get up to a speed where you can run at low RPM in a high gear, the sooner you're using much less gas per mile - so accelerating hard might save gas in the longer term...providing you're doing enough high-speed miles to counteract what you wasted in accelerating hard.
 * But...if you're travelling faster, there is much more aerodynamic drag on the car (drag is proportional to the SQUARE of your speed - so driving half as fast gets you one quarter the amount of drag!) - which has to be paid for in fuel consumption no matter how you drive...so you should drive as slowly as possible to minimize that.
 * But...if you drive more slowly and take a longer time to get to your destination - then you'll be running the air-conditioner, water pump, power steering, etc for longer.
 * But...the energy cost of running the A/C (etc) is more or less the same for small cars and large ones - and for small, fuel-efficient cars it's a big percentage of your fuel bill - for large gas-guzzlers, it's not. So as a proportion of overall mpg, driving quickly so as to minimize journey time is more important in small, efficient cars and may outweigh much of the penalties of increased drag.
 * ...etc, ad nauseam!


 * There are just too many variables - too many different vehicles - too many different situations.


 * Many years ago, to win a bet with a Prius owner - I decided to see what the best MPG I could get out of my car (a MINI Cooper S) was. I drove super-carefully.  With some considerable experimentation, my daily commute went from 36mpg to 45mpg!  But I found that the time I spent driving went up by 10 minutes per day.  At the time, I was earning $50/hour and could work overtime.  So after two weeks, I'd spent 100 minutes driving when I could have been at work, earning overtime. So I wasted $83 in overtime and saved 2 gallons of gas for a total of around $7.  My gas saving driving pattern cost me $76 in lost earnings!  Even if I rate my spare time at $5/hour rather than $50/hour - it's still cheaper to drive fast.


 * For most people, the inconvenience of driving in the most fuel-efficient manner (eg driving on a 75mph freeway at 55mph) totally outweighs the benefits.


 * SteveBaker (talk) 14:02, 15 July 2013 (UTC)

What does "-.-" mean?
What does it mean when someone uses this emoticon: -.- ?

Acceptable (talk) 02:45, 11 July 2013 (UTC)


 * According to List of emoticons, it expresses shame. I've never seen that face on an actual shamed person, but I don't pretend to understand these. InedibleHulk (talk) 02:49, 11 July 2013 (UTC)
 * Our article is sourced to this, which I understand even less. Google Translate says "じと" is "same door". No clue. InedibleHulk (talk) 02:50, 11 July 2013 (UTC)


 * Apparently, (>_<) means "wanna" and "Tsu fiddling". Someone should probably find an English source for this article. InedibleHulk (talk) 02:58, 11 July 2013 (UTC)
 * These emoticons mean what you want them to mean. \\(>..<)// !  KägeTorä - (影虎)  ( TALK )  06:43, 11 July 2013 (UTC)
 * I use it when somebody tells me something stupid or some really bad joke. Alternatively, I take ._. to mean "I'm speechless". ☯ Bonkers The Clown  \(^_^)/  Nonsensical Babble  ☯ 09:06, 11 July 2013 (UTC)
 * I've gotten a few YouTube replies to potentially bad/stupid jokes with that, now that I think of it. Sort of like this look Mr. Burns sometimes shoots Smithers, I guess. The Japanese "shame" might mean "to shame someone", rather than "expressing own shame". Sort of a mix of suspicion and contempt, perhaps. InedibleHulk (talk) 02:19, 12 July 2013 (UTC)


 * "Expresses shame" makes sense to me; writer has their eyes lowered in shame and a small sad mouth. OTOH PC.net says it means stressed. Gandalf61 (talk) 14:11, 11 July 2013 (UTC)
 * （-.-） means looking/watching/staring, sometimes coldly/remotely/cynically. The original form of "じと" might be "じっと". (>_<)  means "Ouch!". Oda Mari (talk) 16:57, 11 July 2013 (UTC)

I and those around me have used this symbol to represent a state of disappointment. I think certain symbols like these are hard to classify because different people give them different meaning. Even if two people share the same meaning, each person might attach a different level of intensity to it. For instance, disappointment at a silly joke that someone told as opposed to levels of disappointment that bring people to livid anger.

216.173.145.47 (talk) 20:31, 11 July 2013 (UTC)
 * Yes, looks like a palmless facepalm to me. μηδείς (talk) 21:58, 11 July 2013 (UTC)
 * Weird. I assumed it was an expression of serenity—perhaps used in reaction to someone being excited, to mean something like "yea, whatever, dude", or "sigh, this is too much to react to". I see now how much it depends on the font. I used Lucida Grande for chat, in which the hyphens are fairly long and low. Pfly (talk) 20:41, 11 July 2013 (UTC)


 * It reminds me of Buddha in meditation. --   Jack of Oz   [Talk]  02:48, 12 July 2013 (UTC)
 * Which Buddha did you see in meditation? There're lots of them, all meaning different things. \\(>..<)//.  KägeTorä - (影虎)  ( TALK )  12:37, 12 July 2013 (UTC)
 * This is the one I had in mind. --   Jack of Oz   [Talk]  23:14, 12 July 2013 (UTC)
 * (-.-).....  KägeTorä - (影虎)  ( TALK )  09:28, 13 July 2013 (UTC)


 * I've seen it used, and use it myself, to indicate a blank stare... as a kind of mild annoyance at whatever someone said. It'd be a little bit like if someone used a bad pun.... -_- would be an appropriate response. Shadowjams (talk) 06:56, 13 July 2013 (UTC)
 * I agree with Shadowjams, it may mean shame in some countries, but when I use it in Canada it is to show being not impressed. 50.101.155.10 (talk) 16:29, 13 July 2013 (UTC)