• Jul 23rd 2009 at 11:37AM
  • 49
Imagine, if you will, taking a sheet of paper and cutting it in half. Now take one of those halves and cut it in half again. Now keep repeating the process. As you keep cutting, the difference in the size of the subsequent pieces gets progressively smaller. This simple example is a demonstration of why continuing to increase the fuel mileage of a vehicle has less and less impact once you get beyond about 35-40 mpg.

Time for a bit of a math lesson. The amount of fuel consumed by vehicle is determined by dividing the number of miles driven by the miles per gallon. With the mileage being in the denominator, as it grows at a linear pace, the overall result gets progressively smaller. This is what's known as a geometric series. One last example: imagine you have a pie that represents the amount of fuel you use to drive a certain distance at a given mileage. Continue reading after the jump to find out what happens to the pie.

Slice the pie in half. Half of the pie represents the fuel used if you double the mileage. At first, each cut is a big deal. You can go from one pie to half a pie by doubling the mpg. But now cut the pie again and again. As you keep going, the difference in the size of the slices gets smaller and smaller. It also gets progressively more difficult to slice the pie. Similarly, improving the efficiency of a car gets more and more difficult. The cost of increasing efficiency rapidly outweighs the savings – both financially and in terms of its effect on the environment – particularly if you are looking for a larger vehicle.

In the United States, where larger vehicles have long been the rule, the benefits of hypermiling a Prius to get from 50 to 60 mpg may be personally satisfying to the individual driver, but in the grand scheme of things, it will have almost no measurable impact. On the other hand, smaller improvements to vehicles that get mileage in the teens and even twenties has a much bigger impact. Another way to understand this phenomenon is to think in terms of gallons per mile (GPM) instead of miles per gallon.

An improvement from 12 to 15 mpg yields a fuel consumption reduction of 200 gallons per year over 12,000 miles. Even going from 21 to 24 mpg saves 71.4 gallons annually. By comparison that 10 mpg increase from 50 to 60 mpg only saves about 35 gallons. That's why, when General Motors introduced its two mode hybrid system, it went for the big bang for buck by applying it to full-size SUVs and pickups. In retrospect, as big truck sales collapsed in 2007-8, the demand for the hybrid versions was too limited. Still, similar reasoning explains why Ford is introducing it's EcoBoost direct injected turbocharged engines starting with the new 3.5-liter V6.

The same phenomena is part of the reasoning behind cash for clunkers. Taking older, lower mileage cars off the and replacing them with vehicles that only get 8-10 mpg more can have a much bigger impact than replacing a 35 mpg car with one that gets 45 mpg. Of course, there are other issues with the specific implementation of cash for clunkers in the U.S., but that's another story. The really important thing in the near term is to make improvements to high volume, high consumption vehicles.

If two million vehicles that currently get 15 mpg were improved to 24 mpg, that would save 267 million gallons of gasoline annually. The American Petroleum Institute estimates that each 42 gallon barrel of crude yielded 19.4 gallons of refined gasoline. That would amount to savings of 13.7 million barrels a year. A similar 9 mpg improvement from 33 to 42 mpg would only save 4.9 million barrels a year. With an additional 2 million new vehicles displacing older vehicles every year, the savings would be compounded. It's certainly important to improve as many vehicles as possible, but with limited resources, the poorest vehicles deserve the biggest efforts. Now, who's got a bit of pie for me?

Mileage (MPG) Amount of fuel used per year (gallons), 12,000 miles Savings per year (gallons), 12,000 miles
3 4,000.00 4,000.00
6 2,000.00 2,000.00
9 1,333.33 666.67
12 1,000.00 333.33
15 800.00 200.00
18 666.67 133.33
21 571.43 95.24
24 500.00 71.43
27 444.44 55.56
30 400.00 44.44
33 363.64 36.36
36 333.33 30.30
39 307.69 25.64
42 285.71 21.98
45 266.67 19.05
48 250.00 16.67
51 235.29 14.71
54 222.22 13.07
57 210.53 11.70
60 200.00 10.53
63 190.48 9.52
66 181.82 8.66
69 173.91 7.91
72 166.67 7.25
75 160.00 6.67
78 153.85 6.15
81 148.15 5.70
84 142.86 5.29
87 137.93 4.93
90 133.33 4.60
93 129.03 4.30
96 125.00 4.03
99 121.21 3.79
102 117.65 3.57
105 114.29 3.36
108 111.11 3.17
111 108.11 3.00
114 105.26 2.84
117 102.56 2.70
120 100.00 2.56

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    • 1 Second Ago
      • 6 Years Ago
      The original Autoblog has covered this subject before, and it got me to thinking: There's always a horsepower and towing battle in the full size truck market. How about a gas mileage battle for a change?

      Attention, GM, Ford, Dodge, and Toyota: Get to work immediately on the 25mpg (or better) truck--make it a hybrid, use alternative fuel, whatever. The first one is probably going to be a short bed with 2WD, but that could work for a lot of fleets and weekend warriors. Work your way up and give us an extended cab 2WD later, then wow us by developing 4x4 and crew cab editions that can do it too.

      We already know that truck makers would rather everyone shell out for their full-sizers instead of compact and mid-sizers--this would be a great way to get that done.
      • 6 Years Ago
      It's been a long time since third grade,thanks for the refresher,ABG.
      If You could somehow come up with a solution for my grammar block,now,that would be useful.
      • 6 Years Ago
      "That's why, when General Motors introduced its two mode hybrid system, it went for the big bang for buck by applying it to full-size SUVs and pickups."

      Really? You think GM did that to reduce total American fuel consumption? What on earth would the benefit to GM be of lower total US fuel use?
      I think they put it in big vehicles because the high (initial) cost of a complex system would be a comparatively small increase in total vehicle cost.
        • 6 Years Ago
        Actually, GM applied the two-mode to large vehicles because it was adapted from a hybrid bus system that GM acquired when GM Powertrain alligned with Allison Transmission in 2003. At the time it was easier to fit it into a larger vehicle than try to cram it into a compact.
      • 6 Years Ago
      Another thought I just had: the knee of the curve is definitely below 30 mpg. As an engineer, I'm thinking 30 mpg would probably be considered low hanging fruit; as in, easily attainable without significant investment of resources. I've always said I would buy a vehicle, on mpg alone, if it hits 75 mpg, or better, in all circumstances. I'm thinking I might be waiting a while. My experience says that sort of efficiency will take considerable investment and cost a whole lot at the register.
      • 6 Years Ago
      Specious, pointless argument that allows idiots who pointlessly drive hulking SUVs to justify continuing to drive them. I am not arguing against people who need them for work.

      We save a hell of lot more gas getting people out of SUVs than making a slightly better SUV.

      When I was a kid before the SUV crazy my parents had 3 kids and drove us around in a BUICK.

      People had bigger families back then and coped just fine with Sedans and wagons. Now we need 5000 lb 4WD bus for 1.8 kids.
        • 6 Years Ago
        SUV's are the modern version of the station wagon. Most of the 1970's station wagons weighed right around 5,000 lbs.

        Personally, I don't mind SUV's. Gas guzzling hogs like the Hummer will kill themselves, I'm just surprised it took as long as it did.

        Models like the 2010 Chevy Equinox, though, which gets 32 mpg on the highway and 22 in the city, are only slightly worse than my 2000 Saturn 1.9L coupe, which averages 23/33.

        One thing this article didn't mention is that 12,000 miles / year is an average, and that cars with better MPG ratings tend to be driven more (which kind of negates their advantage).
        • 6 Years Ago
        That sounds like a another faulty assumption, that people with higher MPG drive more so it negates the advantage.

        It makes more sense to me that people who have long commutes, buy cars with better MPG because it affects their bottom line more directly. It seems like a lot of diesel owners put 30 000+ miles on their cars per year if you read their claims. I seriously doubt buying a diesel makes them drive hundreds of aimless extra miles per week. More likely have hideously long commutes or driving jobs and bought a diesel to save money.

        FWIW I drive a small fairly efficient 4-cyl car and put under 6000 miles per year on it.

        • 6 Years Ago
        Actually, the study was done on vehicle owners with similar commutes.

        • 6 Years Ago
        Quoting your link:
        "these findings has something to do with the fact that high mileage drivers are typically more interested in hybrids than infrequent drivers."

        Again this doesn't sort cause from effect. If you don't drive much you are much less likely to spend significantly more on a vehicle that won't save you much.

        I am not very likely to buy a hyrbid/diesel because I only drive under 6000 miles/year but if I do I am not going to start driving more. I don't drive much because I walk to work year round. I am not going to start driving to work because I changed cars.
      • 6 Years Ago
      That's why GM put it's "two mode" transmission in trucks first, because the of the amount of fuel saved with trucks, versus cars.It seems the media will not give GM credit for doing so or maybe they don't understand it.
      • 6 Years Ago
      Now the only part of this analysis that is missing is layering on top the actual distribution of the cars that people drive by MPG. If 90% of the populace drive cars that get 19 combined MPG and 10% drive cars that get 9 MPG, improvements to the 19 MPG cars will have a demonstrably larger 'bigger picture' impact than similar improvements to the 9 MPG.
      • 6 Years Ago
      kmpl is better than l/100 km. Its better to know how much kilometers its runs in a litre of petrol.

        • 6 Years Ago
        Actually don't we need both

        one to determine travel distance on a given amount of fuel (m/g or km/L)

        and the other to determine the rate of fuel consumption (g/m, g/100 miles or L/100 km)
      • 6 Years Ago
      This is why the European unit of L / 100km is a much better way to measure fuel consumption. Linear.
        • 6 Years Ago
        I also like the European spec of L 100 km. It is much more accurate, as are most things done using the metric system. (Although that's another problem, for another time).
        • 6 Years Ago
        We use L / 100kms in Australia too. This article shows it makes more sense than MPG.

        12 -> 15mpg converts to 19.57 -> 15.66L/100km

        50 -> 60mpg converts to 4.69 > 3.91L/100km

        A lot easier to see what saves more fuel
        • 6 Years Ago
        The Metric system doesn't hold any accuracy advantage over any other system. Jeez, what a ridiculous statement. Either system can measure anything you want to as many decimal places as you'd like.
        • 6 Years Ago

        I don't think Ross was trying to say that the metric system is more accurate, but rather that measuring "fuel consumption" rather than "fuel economy" might be a better idea. He referenced L/100km, meaning liters per hundred kilometers (note the reversal from a miles-per-gallon standpoint). This has nothing to do with metrics versus imperial units but with how you consider fuel use. Do we want to know how much fuel any given car uses going a fixed distance, or do we want to know how far any given car can go on one gallon of fuel?
      • 6 Years Ago
      I think the American mindset got influenced by an orientation of caring more about the time between fill-ups than actual efficiency.

      Part of this is from when fuel is cheap, the fill-up nuissance factor is part of the issue. But I believe back in the 70's during our first 'energy crisis' where gas shortages were real people would have to wait in long lines to get their share of gas. So the longer it lasted the better. And in that respect, mileage is the more immediate story teller.

      Size of the fuel tank matters too. If makers realized that it was time between fill-ups and not actual efficiency that drove customers when gas was cheap, we'd all have bigger gas tanks. Case in point, the H2. It got complaints about fuel efficiency. Anyone buying an H2 knows what they're getting into in terms of efficiency. What they don't realize is that some genius gave it a tinie winie gas tank. And the only way people have to complain about the frequency of needing to fill up is to ping them in the checkbox labelled fuel economy.
      • 6 Years Ago
      Wait until gas is $8-$10. Then a mere 10% improvement will make a lot more sense.
      • 6 Years Ago
      Stop trying to say the most-fuel using version of a vehicle (Flex EcoBoost, Taurus SHO) is somehow a fuel saver.

      You literally cannot get a version of these cars that uses MORE fuel.
        • 5 Years Ago
        "EcoBoost" would have been a great engine in 1991.

        Today we need 3 cylinder fuel economy with 6 cylinder performance.
        - China and India are almost immune to high fuel cost because they aren't running their small business's with 300 HP Pickup Trucks.

        That one fact should scare them into doing something REAL with pickup mpg, before the crisis.

        • 6 Years Ago
        Don't be a bonehead. The article is directly comparing the Ecoboost 3.5L to hybrids. The difference is hybrids are the most efficient versions of the cars in question while the Ecoboost 3.5L is the performance version (and most guzzling in 2 of 3 cases).

        Right now, the Ecoboost has not been used in any eco situation, and ABG trying to imply it has by comparing it to hybrids.

        There is zero eco about a 355HP family sedan.
        • 6 Years Ago
        Geez LS2LS7, can you not get over your blind and tedious love for all things GM.
        The author did not imply Ford was saving fuel with the Ecoboost vs the standard V6 in the Taurus. The implication is that Ford can save more fuel by using a V6 rather than a V8 in larger vehicles (or a V4 instead of a V6) instead of just tweaking the mileage of an already efficient V6 or V4.

        • 6 Years Ago
        LS2, you and so many other people are obsessed with the "Eco" part of EcoBoost, you forget the "Boost" part. Like they say, "V6 like economy with V8 like performance". If anybody is comparing eco-boost to a hybrid, they're crazy, but if you look at it as it is intended, then it is a complete success so far. Just one example is the Lincoln MKS, I can do this with all the other models if I have to, but I don't really see it necessary.

        Lincoln MKS 3.7L V6 274hp 270 ft lbs torque 17/24 MPG FWD 16/23 MPG AWD
        Lincoln MKS 3.5L V6 355hp 350 ft lbs torque 17/25 MPG AWD
        1 more highway mpg vs. the FWD version and 1 city and 2 highway mpg vs. the AWD would be the "Eco" part, as well as the other things like reduced emissions especially on cold starts. The 80 hp and ft lbs of torque would be the "Boost" part.
        It's exactly as they say, V6 like economy (better in this case), V8 like performance.
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