# Diesel autos are a illusion of increased efficiency **URL:** \ **Category:** Great Debates\ **Created:** [June 5, 2011, 8:30pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429 "2011-06-05T20:30:11Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 5, 2011, 8:30pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/1 "2011-06-05T20:30:11Z")
Many toute that diesel should be a choice motor fuel over gas as it is more efficient and a viable alternative to hybrids and other higher efficiency auto. I very much doubt this when all things are considered. To get a accurate measure you would have to decrease a diesel auto’s mpg by perhaps 10% to get the accurate equivalent gas powered MPG (if your diesel gets 40mpg that is like a 36 mpg gas car). While 40 mpg’s sounds good, it is just a marketing gimmick more then actually energy savings and is no savings at all in terms of energy or price for fuel. I would like to hear others input. 1 - Cans pack more efficient then bottles. Diesel fuel has more energy then gas per gallon, but it all comes from the same source which is the energy we pump out of the ground. It is just packed differently. In terms of energy pumped out of the ground 1 gallon of diesel is not equal to 1 gallon of gas so you can’t directly relate MPG’s. The price of diesel fuel also reflects this, so there is no net energy savings or money savings. 2 - Diesel engines do operate at higher compression, which does increases efficiency, but the limit is really artificially imposed as higher compression means higher NOx emissions, which gas cars are restricted in NOx, while diesels are not, giving diesels a artificial advantage. We can expect this advantage to go away of there are much more diesel cars and the emission requirements are imposed on them. We can increase gas engine efficiency by increasing compression or by using it in a modified diesel cycle with gas as the fuel, which adds the possibility of raising it even more. So presently any efficiency gain diesel has over gas is at the expense of harm to health and environment. 3 - Diesel fuel has more carbon, so it contributes more to the release of CO2 - though I’m not sure this holds if you consider the entire cycle. Can I say now that diesel fueled autos that toute environmental benefits and high MPG’s is nothing but a lie told by marketing departments to the potential buyers of said vehicles? ---
**Author:** ![Ravenman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ravenman/32/2929_2.png) [@Ravenman](https://boards.straightdope.com/u/Ravenman)\ **Post date:** [June 5, 2011, 8:42pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/2 "2011-06-05T20:42:55Z")
> [@kanicbird](#): > > To get a accurate measure you would have to decrease a diesel auto’s mpg by perhaps 10% to get the accurate equivalent gas powered MPG (if your diesel gets 40mpg that is like a 36 mpg gas car). I don’t understand this at all. Please explain again. > [@](#): > > While 40 mpg’s sounds good, it is just a marketing gimmick more then actually energy savings and is no savings at all in terms of energy or price for fuel. Are you saying that when I do on a road trip and average about 47 mpg on the freeway in my diesel, that I’m not actually getting that mileage? Or that my mileage of about 33 mpg in stop-and-go city traffic isn’t real? Or that I am actually filling up my car more often than once every 475 miles? I’m really having a hard time understanding the points you wish to make. ---
**Author:** ![Mr\_Smashy](https://avatars.discourse-cdn.com/v4/letter/m/2bfe46/32.png) [@Mr\_Smashy](https://boards.straightdope.com/u/Mr_Smashy)\ **Post date:** [June 5, 2011, 8:47pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/3 "2011-06-05T20:47:10Z")
Seems to me like you’re overthinking this. Say again what the justification is for an off-the-cuff reduction of 10% of the listed MPG? If it’s because diesel uses 10% more energy to produce, you didn’t provide any cites. You might find [this page](http://www.eia.gov/oog/info/gdu/gasdiesel.asp) interesting; looks like the cost of the crude oil, which is the biggest constraint going forward is [$2.13 to make diesel](http://ask.metafilter.com/86901/Which-costs-more-to-make-diesel-or-gasoine) vs $2.09 for gasoline. But there are a million variables, including the type of crude we’re talking about (West Texas Intermediate?) and the type of refinery (topping, hydrocrackers, reforming, cokers). Some say it’s a 20% discount in mileage, some say zero. Edit to add: **Raven** apparently beat me to the punch. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 5, 2011, 8:58pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/4 "2011-06-05T20:58:33Z")
> [@Ravenman](#): > > I don’t understand this at all. Please explain again. > > Are you saying that when I do on a road trip and average about 47 mpg on the freeway in my diesel, that I’m not actually getting that mileage? Or that my mileage of about 33 mpg in stop-and-go city traffic isn’t real? Or that I am actually filling up my car more often than once every 475 miles? > > I’m really having a hard time understanding the points you wish to make. Simply stated you can’t compare gas to diesel fuel on a per gallon basis. That is comparing apples and oranges. When you say you get 47 mpg that is not the same as a gas auto that gets 47 mpg. In this case the gas car is much more efficient (from a energy/mile standpoint) though the mpg is the same. The question is how much to discount the diesel MPG’s to equate the 2 fuels. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 5, 2011, 9:06pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/5 "2011-06-05T21:06:14Z")
> [@Ravenman](#): > > Are you saying that when I do on a road trip and average about 47 mpg on the freeway in my diesel, that I’m not actually getting that mileage? For this car was there a gas option, if so what is the millage for that? ---
**Author:** ![Mr\_Smashy](https://avatars.discourse-cdn.com/v4/letter/m/2bfe46/32.png) [@Mr\_Smashy](https://boards.straightdope.com/u/Mr_Smashy)\ **Post date:** [June 5, 2011, 9:22pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/6 "2011-06-05T21:22:20Z")
> [@kanicbird](#): > > 2 - Diesel engines do operate at higher compression, which does increases efficiency, but the limit is really artificially imposed as higher compression means higher NOx emissions, which gas cars are restricted in NOx, while diesels are not, giving diesels a artificial advantage. We can expect this advantage to go away of there are much more diesel cars and the emission requirements are imposed on them. We can increase gas engine efficiency by increasing compression or by using it in a modified diesel cycle with gas as the fuel, which adds the possibility of raising it even more. So presently any efficiency gain diesel has over gas is at the expense of harm to health and environment. > > 3 - Diesel fuel has more carbon, so it contributes more to the release of CO2 - though I’m not sure this holds if you consider the entire cycle. And on these two points, where are you getting your data? You don’t list any cites, and the 5 seconds of google came up with [this](http://uk.answers.yahoo.com/question/index?qid=20060926082929AA4DvqB) > [@](#): > > The five main emissions for petrol and diesel cars are: > > Carbon Dioxide (CO2) > > Carbon Monoxide (CO) > > Nitrogen Oxides (NOx) > > Hydrocarbons > > Particulates > Of these five, a diesel car is better than a petrol car with three of them, about the same with one, and worse with one. If you are worried about global warming (CO2), then diesel is better. Nitrous Oxide? According to this source, gas is better for the first 50k miles of a car’s life, then it’s even, then diesel is better (meaning gas cars cause more smog, on a per unit basis). Worried about cancer and hydrocarbons? > [@](#): > > Hydrocarbon emissions are contained in petrol engine emissions much more than in diesel engine emissions. I’m not sure why you even care about raw efficiency, per se… natural gas is way more inefficient but burns cleaner for the environment. Seems to me, a better question would be, which is safer for the environment, which is better bang for the buck economically, which has a lower TCO. ---
**Author:** ![Ravenman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ravenman/32/2929_2.png) [@Ravenman](https://boards.straightdope.com/u/Ravenman)\ **Post date:** [June 5, 2011, 9:47pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/7 "2011-06-05T21:47:04Z")
> [@kanicbird](#): > > Simply stated you can’t compare gas to diesel fuel on a per gallon basis. That is comparing apples and oranges. When you say you get 47 mpg that is not the same as a gas auto that gets 47 mpg. I’ve written this post like four times, because the point you seem to be making is so weird I don’t know how to respond to it. Yes, when I get 47 mpg, my car is different than a gas-powered car that gets 47 mpg. The gas powered car either has to be a hybrid or smaller and lighter than the car I drive in order to get the same mileage. Is this the point you’re trying to make? That you have to make hybrids or smaller gas-powered cars to match diesels’ economy? > [@](#): > > In this case the gas car is much more efficient (from a energy/mile standpoint) though the mpg is the same. The question is how much to discount the diesel MPG’s to equate the 2 fuels. What? No! When there are identical cars that are powered by gas or diesel, the diesel is more fuel efficient. [See the very bottom of this page.](http://www.vw.com/en/models/jetta/trims-specs.html#/tab=0d1fbce9887e4e2e4a6dc09a77b0a834) Or scroll down about 80% of the way down [these](http://www.bmwusa.com/Standard/Content/Vehicles/2011/3/335iSedan/Features_and_Specs/335iSedanSpecifications.aspx) [two](http://www.bmwusa.com/Standard/Content/Vehicles/2011/3/335dSedan/Features_and_Specs/335dSedanSpecifications.aspx) pages. The diesels on those pages get about 30% better mileage. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 5, 2011, 10:11pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/8 "2011-06-05T22:11:44Z")
> [@Ravenman](#): > > I’ve written this post like four times, because the point you seem to be making is so weird I don’t know how to respond to it. > > Yes, when I get 47 mpg, my car is different than a gas-powered car that gets 47 mpg. The gas powered car either has to be a hybrid or smaller and lighter than the car I drive in order to get the same mileage. Is this the point you’re trying to make? That you have to make hybrids or smaller gas-powered cars to match diesels’ economy? No quite the opposite point. That your 47 mpg of diesel is deceiving you into thinking you are matching hybrid car efficiencies, when all you are doing is burning a more compact fuel creating a illusion that you are getting better efficiencies then you really are while getting no better mileage then a similar gas powered car in terms of energy used. The page you cite gives a millage of 33 for gas and 42 for diesel, though the gas powered car is quicker, so that would seem to skew the numbers a bit in favor of the diesel a tad. If you equate the performance of the 2 cars you would have to up the power of the diesel (or lower the power of the gas). This would narrow the mpg’s between them to less then 30%. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 5, 2011, 10:21pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/9 "2011-06-05T22:21:06Z")
**Mr Smashy** your cite states > [@](#): > > Carbon dioxide emissions are directly proportional to fuel consumption, and as diesel cars use 30 to 40% less fuel, they emit 30 to 40% less carbon dioxide than petrol cars. I don;t think you can make that claim because the reactions are different: Diesel 4C12H23 + 71O2 –\> 48CO2 + 46H2O Gas 2C8H18 + 25O2 ~\> 16CO2 + 18H2O Gasoline produces less CO2 and more H2O on a percent bases then diesel. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [June 5, 2011, 10:22pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/10 "2011-06-05T22:22:17Z")
> [@kanicbird](#): > > Can I say now that diesel fueled autos that toute environmental benefits and high MPG’s is nothing but a lie told by marketing departments to the potential buyers of said vehicles? Not really. You are quite correct that diesel fuel contains slightly more energy per unit volume than gasoline and so a direct mpg comparison isn’t exactly fair, but it’s far from the end of the story. The compression ratio is key, and the capability of higher compression ratios is fundamental to the way diesels work. Gasoline engines compress a pre-mixed charge of air and fuel that is prone to detonation if it’s compressed too much before ignition, or will even auto-ignite if it’s really compressed. The NO2 limits are a secondary problem - catalytic converters take care of the NO2 anyway. Diesels only compress a charge of air so there is no problem of autoignition, and when the fuel is injected at the top of the cycle it is not homogenously mixed so there is no possibility of detonation. This allows diesels to have a substantially higher compression ratio, which more importantly means they have a substantially higher _expansion_ ratio than gasoline engines. This means the down-stroke of the diesel’s pistons extracts more energy from the combustion products before they exhaust. (This is more than enough to compensate for the diesel’s disadvantage of burning fuel through a longer proportion of the stroke.) You can extract even more energy from the expanding gases with a turbocharger. In a diesel, this energy feeds back into the engine through higher intake pressure and actually adds to the overall engine efficiency, as well an increasing the engine’s power. In a gasoline engine, a turbo also adds energy through higher intake pressure but the efficiency gain is lost because the compression ratio is more limited in turbocharged engines. In general, turbos add power but not efficiency to gasoline engines, in diesels they add both. A final factor is the way the engines modulate their power. A gasoline engine generates low and intermediate power levels by throttling - forcing the engine to suck air through a small hole. In effect the engine is being forced to act as an air pump against a high resistance, which wastes energy. A diesel engine isn’t throttled - it simply injects less fuel and so doesn’t lose efficiency at low power. This is a big deal for city driving. ---
**Author:** ![Ravenman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ravenman/32/2929_2.png) [@Ravenman](https://boards.straightdope.com/u/Ravenman)\ **Post date:** [June 5, 2011, 10:48pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/11 "2011-06-05T22:48:24Z")
[quote=“kanicbird, post:8, topic:584429”] No quite the opposite point. That your 47 mpg of diesel is deceiving you into thinking you are matching hybrid car efficiencies, when all you are doing is burning a more compact fuel creating a illusion that you are getting better efficiencies then you really are while getting no better mileage then a similar gas powered car in terms of energy used. [quote] It makes no sense to complain that diesel has more energy – to use another analogy, it’s like saying that one can’t compare the sweet taste of high fructose corn syrup and sugar. The whole point of buying HFCS is that you don’t need as much of it to achieve the desired result. > [@](#): > > The page you cite gives a millage of 33 for gas and 42 for diesel, though the gas powered car is quicker, so that would seem to skew the numbers a bit in favor of the diesel a tad. If you equate the performance of the 2 cars you would have to up the power of the diesel (or lower the power of the gas). This would narrow the mpg’s between them to less then 30%. Equate the performance? What are you talking about? Why does anyone need to alter diesels and gas engines in order to make them equal? The fact that they aren’t equal is the whole point. Want to accelerate quickly? Get a gas engine. Want lots of torque? Buy a diesel. ---
**Author:** ![Martin\_Hyde](https://avatars.discourse-cdn.com/v4/letter/m/47e85d/32.png) [@Martin\_Hyde](https://boards.straightdope.com/u/Martin_Hyde)\ **Post date:** [June 5, 2011, 11:17pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/12 "2011-06-05T23:17:27Z")
What’s weird is I’ve seen a few threads like this on diesel and it’s very confusing because the diesel detractors are always all over the place. If all we want to talk about is the household economics of diesel then if you compare two identical cars one diesel powered and one gasoline powered the diesel car will cost less to operate on a yearly basis (and will cost more upfront.) If you go to [fueleconomy.gov](http://fueleconomy.gov) you can see for yourself. A 4 cyl, 2.0L Volkswagen Jetta automatic running on gasoline has a projected annual fuel cost of $2310. A 4 cyl, 2.0L Volkswagen Jetta automatic running on diesel has a projected annual fuel cost of $1764. So the diesel’s fuel cost is about 23% off the cost of the gasoline. Looking at Volkswagen’s price the base price of the gasoline model is $17,095. The base price of the diesel model is $24,095. That price difference means if you realize the estimated annual fuel savings above it would take almost 13 years of driving to justify the higher upfront cost. However the actual cost of fuel being put into the car isn’t really debatable, diesels are cheaper to operate and move the car on less raw energy per mile than a gas car. Part of the price difference between models is that diesel engines are just more expensive, and part of it is new diesels have become extremely popular because people are panicking over increased fuel prices and dealers and manufacturers aren’t really being forced to negotiate much to move diesels. ---
**Author:** ![Martin\_Hyde](https://avatars.discourse-cdn.com/v4/letter/m/47e85d/32.png) [@Martin\_Hyde](https://boards.straightdope.com/u/Martin_Hyde)\ **Post date:** [June 5, 2011, 11:21pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/13 "2011-06-05T23:21:19Z")
Also before a diesel supporter rails against me, understand that [fueleconomy.gov](http://fueleconomy.gov) makes assumptions of 45% highway and 55% city driving, and 15000 miles per year. Obviously in personal situations where those numbers are different the number of years a diesel’s cheaper fuel cost will offset its higher sticker price can be much less than the almost 13 years it would take at the [fueleconomy.gov](http://fueleconomy.gov) assumed cost. Also according to the same website the carbon footprint of the diesel is 6.2 tons a year and the automatic is 7.5 tons. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 5, 2011, 11:27pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/14 "2011-06-05T23:27:36Z")
> [@Martin\_Hyde](#): > > Also according to the same website the carbon footprint of the diesel is 6.2 tons a year and the automatic is 7.5 tons. ??? do you mean gasoline is 7.5 tons? ---
**Author:** ![Martin\_Hyde](https://avatars.discourse-cdn.com/v4/letter/m/47e85d/32.png) [@Martin\_Hyde](https://boards.straightdope.com/u/Martin_Hyde)\ **Post date:** [June 5, 2011, 11:33pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/15 "2011-06-05T23:33:53Z")
> [@kanicbird](#): > > ??? do you mean gasoline is 7.5 tons? Yes. Of course the easiest thing to do would be to go to [www.fueleconomy.gov](http://www.fueleconomy.gov) and look at the information for yourself as then you would not have to interpret it through my typos and such ;). As for the efficiency of the actual engine, based on simplistic calculations I’m seeing that one gallon of gasoline has 115,400 net BTU and one gallon of diesel has 128,700 net BTU. Using the MPGs of the diesel vs gasoline Jettas traveling on the highway the diesel Jetta can move the car one mile using 3064 BTU while the gasoline powered Jetta uses 3979 BTU to move the car one mile. So even accounting for the greater energy density of diesel fuel, the diesel engine powered automobile is still moving the car over a comparable distance in a comparable manner while expending less energy–which suggests to this layman with no engineering or scientific expertise that the diesel engine is thus operating intrinsically more efficiently than the gasoline engine. Of course I’m basing this on 5 minutes of research on Google to get these numbers so there may be calculations of efficiency that I’m unaware of or various principles I’m ignoring in these calculations. ---
**Author:** ![Mr\_Smashy](https://avatars.discourse-cdn.com/v4/letter/m/2bfe46/32.png) [@Mr\_Smashy](https://boards.straightdope.com/u/Mr_Smashy)\ **Post date:** [June 5, 2011, 11:49pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/16 "2011-06-05T23:49:59Z")
> [@kanicbird](#): > > **Mr Smashy** your cite states > > I don;t think you can make that claim because the reactions are different: > Diesel > 4C12H23 + 71O2 –\> 48CO2 + 46H2O > Gas > 2C8H18 + 25O2 ~\> 16CO2 + 18H2O > > Gasoline produces less CO2 and more H2O on a percent bases then diesel. Well, I admit that was a very quick search. Here’s another cite, this time from [EPA](http://www.epa.gov/oms/climate/420f05001.htm). Let me cut to the chase: > [@](#): > > Finally, to calculate the CO2 emissions from a gallon of fuel, the carbon emissions are multiplied by the ratio of the molecular weight of CO2 (m.w. 44) to the molecular weight of carbon (m.w.12): 44/12. > > CO2 emissions from a gallon of gasoline = 2,421 grams x 0.99 x (44/12) = 8,788 grams = 8.8 kg/gallon = 19.4 pounds/gallon > > CO2 emissions from a gallon of diesel = 2,778 grams x 0.99 x (44/12) = 10,084 grams = 10.1 kg/gallon = 22.2 pounds/gallon On average, that says that gas is ~10% less CO2 than diesel. Since diesel engines are generally more fuel efficient over equal gas models (the 33 mpg for gas vs 42 mpg for diesel), you cannot say with any degree of credibility that diesels kick out more CO2 per mile, on a per unit basis, than gas models. If you have a cite that says otherwise, I’ll read it, but so far all your postings seem to be pretty fact-free. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 5, 2011, 11:56pm UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/17 "2011-06-05T23:56:53Z")
> [@Martin\_Hyde](#): > > Yes. Of course the easiest thing to do would be to go to [www.fueleconomy.gov](http://www.fueleconomy.gov) and look at the information for yourself as then you would not have to interpret it through my typos and such ;). > > As for the efficiency of the actual engine, based on simplistic calculations I’m seeing that one gallon of gasoline has 115,400 net BTU and one gallon of diesel has 128,700 net BTU. Using the MPGs of the diesel vs gasoline Jettas traveling on the highway the diesel Jetta can move the car one mile using 3064 BTU while the gasoline powered Jetta uses 3979 BTU to move the car one mile. So even accounting for the greater energy density of diesel fuel, the diesel engine powered automobile is still moving the car over a comparable distance in a comparable manner while expending less energy–which suggests to this layman with no engineering or scientific expertise that the diesel engine is thus operating intrinsically more efficiently than the gasoline engine. > > Of course I’m basing this on 5 minutes of research on Google to get these numbers so there may be calculations of efficiency that I’m unaware of or various principles I’m ignoring in these calculations. The carbon emissions also seem in line with this as well. So according to that cite diesels are more efficient. But also you can not equate gas mpg’s with diesels mpg as they are different fuels with different energy contents and sell at different costs. Due to the more energy in diesel fuel the numbers are skewed in favor of the diesel more then just efficiency gains alone. ---
**Author:** ![Mr\_Smashy](https://avatars.discourse-cdn.com/v4/letter/m/2bfe46/32.png) [@Mr\_Smashy](https://boards.straightdope.com/u/Mr_Smashy)\ **Post date:** [June 6, 2011, 12:02am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/18 "2011-06-06T00:02:40Z")
> [@kanicbird](#): > > The carbon emissions also seem in line with this as well. So according to that cite diesels are more efficient. > > But also you can not equate gas mpg’s with diesels mpg as they are different fuels with different energy contents and sell at different costs. Due to the more energy in diesel fuel the numbers are skewed in favor of the diesel more then just efficiency gains alone. So Kanic, are you retracting part of your OP where you stated > [@](#): > > 3 - Diesel fuel has more carbon, so it contributes more to the release of CO2 - though I’m not sure this holds if you consider the entire cycle. By the way, why can’t you compare MPGs? Of course they are different fuels. You can compare the calories in cookies vs the calories in broccoli, but they are different fuels too, at different price points, etc. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [June 6, 2011, 12:29am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/19 "2011-06-06T00:29:25Z")
> [@Mr\_Smashy](#): > > So Kanic, are you retracting part of your OP where you stated It does have more carbon, but it appears from the cite that because of the efficiency there is overall less fuel burned leading to lower carbon footprint of the car. So yes I will. > [@](#): > > By the way, why can’t you compare MPGs? Of course they are different fuels. You can compare the calories in cookies vs the calories in broccoli, but they are different fuels too, at different price points, etc. It’s a bit deceptive. You may be able to make a car that runs on liquid hydrogen that gets 170 mpg while the equivalent gas car gets 30. It would not be a fair way to compare efficiency to do it mpg. This is a exaggeration to show a point that if you compare it by mpg you are going to come up with a unfair advantage to the fuel with the higher energy content. Diesel is fairly close enough that most people don’t look, though it gives a non negligible artificially high mpg to diesels. My question is how much? Electric cars are given a mpg equivalent of gasoline, what i am stating is that diesel needs this also. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [June 6, 2011, 12:47am UTC](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429/20 "2011-06-06T00:47:49Z")
The problem is it depends on the variable. NOx? CO2? Particulates? Total cost of ownership over the usual five years? [Next page](https://boards.straightdope.com/t/diesel-autos-are-a-illusion-of-increased-efficiency/584429.md?page=2)