On the Chevy truck, and any other car.

"Spanfeller is a twat" (theaspiringengineer) 07/06/2018 at 13:13 • Filed to: None

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“ The difference in the mpg ratings doesn’t reflect the actual difference in fuel consumption the way that gallons/100 miles does, pure and simple. (...) the EPA should promote this system too. Sure, it’s trivial to calculate the difference, but with the current emphasis that car manufacturers, government agencies and consumers are placing on fuel economy, we need to change our way of thinking from high mileage to low consumption”

!!!error: Indecipherable SUB-paragraph formatting!!!

In Mexico it’s a known strategy of marketing to use km/l rather than l/100km to make a car appear more efficient than it actually is.

RamblinRover Luxury-Yacht > Spanfeller is a twat 07/06/2018 at 13:40

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Neither is an ideal solution. The v/d formula falls apart with thirsty vehicles because variations in efficiency are magnified to the point that differences may be claimed that don’t really exist in normal use. If a car normally gets 16mpg and matches its EPA cycle, while a competitor gets 15 “real world” and rates at 17, they are described as 6.25 and 5.88g/100mi, which makes the second sound much better than it actually will be - and thirsty vehicles quite often experience larger differences between test cycle numbers and practical results. Fairly small percentage-based changes in running conditions are magnified out of relevance, when the real world may betray expectations. The perspective that it allows in such a case is useful.

On the other hand, the d/v formula falls apart with efficient vehicles - which I think is what you’re highlighting. Two vehicles with 45mpg and 50 mpg (2.22 and 2 gal/100 mi) are effectively reflected as having very little difference in v/d form, but the numbers sound significant in d/v.

Either is fairly easy to use in mental arithmetic to calculate expense of traveling. My own familiarity is with d/v, so I’m inclined to think it’s easier, but I don’t know how one would do a blank slate empirical comparison.

jimz > Spanfeller is a twat 07/06/2018 at 13:55

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Ford puts both on the Monroney, though obviously with emphasis on the EPA figures in mpg.

Spanfeller is a twat > RamblinRover Luxury-Yacht 07/06/2018 at 13:59

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“ thirsty vehicles quite often experience larger differences between test cycle numbers and practical results. ”

But there you’re arguing about EPA test cycles v. math..... which I agree makes the whole system harder to judge. How many km I get out of my tank depends very much on where I’m driving, how I’m driving, and which accessories my car has running. (A car that is driving in the desert with the A/C on max doing stop and go traffic isn’t going to be efficient compared to the same car driving in a straight road, at 60km/h, without any accessories on, with a ambient temperature of 10C)

Spanfeller is a twat > jimz 07/06/2018 at 14:02

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Yes, because MPG is easier to brag about.

Also why gallons, or miles?

Either way, the US should change to metric.

RamblinRover Luxury-Yacht > Spanfeller is a twat 07/06/2018 at 14:09

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Regardless whose cycle it is, it will remain a problem for systems that necessarily compare relatively unstressed systems to highly stressed, highly managed ones to give results with meaning in the real world. The first starts at less practical efficiency, but is more antifragile, the second... boy howdy. The big advantage here of using d/v is that it downplays the magnitude of such a difference where it matters - a claim (EPA-backed or not) to magic efficiency wed to attractive performance numbers - a claim that is effectively a lie.

I do have a problem with the PM excerpt, in that it wishes to suggest how *I choose* to conduct my thinking. Since I don’t feel faint at the prospect of *using* an additional two gallons in two weeks, and prefer to think in terms of how much value *in a vehicle* I’m getting for my money (through the mechanism of a vehicle that is efficient), I would like to tell that article to fuck right off. PM has not earned the right to be my thought police, however passive-aggressively.

jimz > Spanfeller is a twat 07/06/2018 at 14:14

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we tried.  people just shot at the road signs. 

Spanfeller is a twat > RamblinRover Luxury-Yacht 07/06/2018 at 14:14

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Um... If I understand correctly your comment, you think MPG gives a better rating than g/100m because it represents mileage and not consumption?

Spanfeller is a twat > jimz 07/06/2018 at 14:15

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God dammit.

Because of the US, a few sectors in Mexico still operate in imperial (many mechanics use lb-ft for torquing bolts, or use PSI to measure tire pressure, cuin to measure displacement). It makes me rattle a bit.

WRXforScience > Spanfeller is a twat 07/06/2018 at 14:18

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People are bad at math, and most vehicle purchase are done irrationally (or using flawed logic).

The Popular Mechanics argument is that people can’t figure out a percent difference (14mpg-21mpg is a 50% improvement, 21mpg-28mpg is a 33% improvement, and 28mpg-35mpg is a 25% improvement even though all are a 7mpg increase).

Family sizes have gone down while vehicle sizes have gone up. CAFE fuel standards are based on vehicle footprint so the bigger a vehicle is the worse its mileage is allowed to be. It is easier to make vehicles bigger than more efficient (plus you can use that as a selling point). We’ve (the US) incentivized   buying bigger vehicles for nearly 20 years, we shouldn’t be surprised that they’ve become more popular.

The problem isn’t that people can’t figure out which vehicles are more efficient, the problem is that people don’t really care. Fuel costs are spread out over long enough periods of time that people do not accurately gauge them in the overall cost of a vehicle.

A Mercedes S550 gets an average of 15mpg, it’s competitor the Tesla Model S costs the same and could be driven for free (but with most electricity rates it looks more like an equivalent of about 45mpg is fair for comparison’s sake). So, the fuel costs for the Mercedes are 3x the cost of the Tesla. Let’s assume that the lifespan of both is 150, 000miles, so the Mercedes uses 10,000gal of gas (it takes premium, so let’s say $3/gal) for a fuel cost of $30,000. The Tesla looks like it would cost about $10,000 to fuel, for a savings of $20,000 or about 20% of the purchase price of the vehicles (they’re both around $ 100k).

More pedestrian, Ford F- 150 gets about 16mpg while the Ford Taurus gets 22mpg (data from F uelly). Both have similar pricing (the F-150 can get much higher), and both can seat 5 people. Fuel costs for the F-150 come out to about $28k, and only $20k for the Taurus. That’s about 30% of the initial vehicle cost.

The SUV to car comparison is about the same for similar pairings, people are paying an extra 20-30% in fuel costs for no benefit because the government accidentally incentivized SUV’s over cars (Minivans and SUV’s are a little harder to compare since only full size SUV’s seat 7, which greatly skews the data in favor of vans, but I’d assume that most vans are really for families of 4-5 which gives a similar savings for vans as the cars get).

TLDR: You can save about $10k by buying a car or van instead of an SUV, but you won’t because of reasons.

Spanfeller is a twat > WRXforScience 07/06/2018 at 14:27

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Yes, some consumers are irrational, I specially don’t understand trucks.

RamblinRover Luxury-Yacht > Spanfeller is a twat 07/06/2018 at 14:28

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I think that it gives a better sense of a vehicle’s efficiency on the lower end of the scale even though the total potential expense variation is greater, and grants a certain amount of perspective in interpreting the very “noisy” set of possible results for an inefficient car.

PM seems to think that if the huge expense difference on a long commute between a 16mpg car and an 18mpg car is driven home by using a different system, it will help people make wiser decisions - failing to realize:

(a) These people have much more efficient choices available in most cases already

(b)The purported difference may not exist

(c)As these people have a larger portion of their budget allotted to fuel expense already, they are more likely to be able to afford such a difference

(d)Using a smaller volume of fuel (being more use-conscious) is not an all-redeeming social value

The thing is, people chasing an ever-more-efficient vehicle have no need to favor v/d for “consciousness” reasons, because the v/d rating reflects a much smaller difference for a highly-efficient car. A car goes from running on pocket change to *slightly less* pocket change.

If that’s not what Popular Mechanics is thinking, and they think it’s meaningful in the ~30mpg section of the chart, they are innumerate, because that is the area of the chart in which the relation between the two systems is most linear. Read: not given to advantage one or the other . Unless the whole purpose behind the adoption is to claim virtue, by saying “I care about how much of Earth’s precious fuel my car uses!” rather than “this car will allow me to go a long distance cheaply”.

I claim the freedom to say the latter.

Spanfeller is a twat > RamblinRover Luxury-Yacht 07/06/2018 at 14:39

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Well, maybe MPG could still be used in heavy duty or high performance applications where they noise you describe would cause unjust differences between two cars that will have similar results.

However, people care about money, in the US more than 80% of people commute by car! and in the same way a Camry dent is a thing, so is not wanting to go dump fuel int your car. be it for enviromental reasons (which in most cases are hypocritical ) or be it for economic differences that for some might be very important. As a measurement for consumers, maybe having both figures is better, but as JimZ posted, Ford still gives more space to MPG than G/100M, and that’s because of the exponential relationship they have.

RamblinRover Luxury-Yacht > WRXforScience 07/06/2018 at 14:52

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I agree with many of your points, but I do have one quibble:

It is easier to make vehicles bigger than more efficient (plus you can use that as a selling point).

...is only somewhat true, because the weight of the car increases relative to (say) length at an exponent greater than 1. The necessary strength of the body elements to remain rigid must increase, which makes for more weight per linear foot. Width has some of the same difficulties, with a greater impact on drag, which is murderous on efficiency.

Therefore, depending on the weight of the footprint standard, a class of cars can be absolutely slaughtered by efficiency reqs beyond any allowance for size. One major reason for there being as few full size sedans anymore.

MasterMario - Keeper of the V8s > Spanfeller is a twat 07/06/2018 at 15:20

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WHAT? I CAN’T HEAR YOU OVER MY 454 CUBIC INCHES OF FREEDOM!

MPG!? YOU MEAN GALLONS PER MILE?

Future next gen S2000 owner > Spanfeller is a twat 07/06/2018 at 15:21

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I was working on a project that was completed during the attempted transition. Really weird seeing xx thousand mm with inches in pare nthesis on design drawings.

Spanfeller is a twat > Future next gen S2000 owner 07/06/2018 at 15:23

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I think there’s still a highway in Arizona that uses km, but otherwise I think the effort was minimal. At least within the science community they use metric

Future next gen S2000 owner > Spanfeller is a twat 07/06/2018 at 15:23

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T he lack of a common root would solve a lot of issues. If a foot was 10 inches. Or is there weren’t two pints to a quart and 4 quart to a gallon. Or 5280 feet to a mile. The mind numbing randomness between conversions is what people hate about the English system.

Spanfeller is a twat > Future next gen S2000 owner 07/06/2018 at 15:26

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yes. It would help. But the wor ld would still be ran with two systems.
You guys did take over cars though... I think all car makers quote in HP rather than kW

just-a-scratch > Future next gen S2000 owner 07/06/2018 at 15:45

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Or we could use a much better number system and leave both the English/imperial system and metric system behind.

Duodecimal FTW!

BASE TWELVE! BASE TWELVE! BASE TWELVE! BASE TWELVE!

gogmorgo - rowing gears in a Grand Cherokee > RamblinRover Luxury-Yacht 07/06/2018 at 15:54

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The 2018 Suburban is two feet longer than the 2018 Tahoe, but only weighs 200lbs more in similar spec (LT, 4x4) . The two vehicles are effectively short and long wheelbase versions of the same SUV. (Specs sourced from Edmonds.com) This difference accounts for EPA ratings of 15/22 for the Suburban and 16/22 for the Tahoe. Both vehicles have a combined rating of 18. The whole-number figures hide precision, but the combined number being the same makes me think there’s closer to 0.5 mpg than a whole 1.0 mpg difference between the two on the city loop, and highway loop is likely a neglige able difference due to the aerodynamic similarities between the two. (I’m not an aerodynamicist, but I’d wager the longer S uburban would perform better). To further that theory I'll point out that subtracting the 4x4  from either vehicle subtracts roughly the same weight, but while the Suburban on city loop increases to 16mpg, the Tahoe stays at 16. Highway loop both go to 23, which will be down to the lower 2wd ride height.

Adding size to a vehicle isn’t necessarily  a huge increase in mass. Most of the “size” added is empty interior space, and the bulk of added weight would come along if the extra size adds another row of seats.

Future next gen S2000 owner > just-a-scratch 07/06/2018 at 15:58

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Why base 12?

RamblinRover Luxury-Yacht > gogmorgo - rowing gears in a Grand Cherokee 07/06/2018 at 16:05

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Interesting. That’s definitely an exception to the general rule I’d propose. I would guess that both have enough surplus stiffness to the frame that they can get away with literally just adding length, with no adjustment to doors, etc. in a way that would bulk it up.

I wonder how much weaker the Suburban’s roof is in rollover?

gogmorgo - rowing gears in a Grand Cherokee > RamblinRover Luxury-Yacht 07/06/2018 at 17:03

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That would be an interesting comparison. Something I did notice was that the Suburban was rated at 8000lbs towing capacity, but that the Tahoe got 8400lbs, which could suggest a reduction in rigidity, but I suspect has more to do with increased weight on rear suspension and a longer rear overhang pushing the tongue weight further off vehicle centre.

just-a-scratch > Future next gen S2000 owner 07/06/2018 at 17:05

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It has so many more factors. It makes a lot of computation much easier.

Here are a couple search results that will explain better than I do.

https://io9.gizmodo.com/5977095/why-we-should-switch-to-a-base-12-counting-system

http://www.dozenal.org/