Slashdot Mirror

← Back to Stories (view on slashdot.org)

Why US Mileage Ratings Are So Inaccurate

Posted by timothy on from the if-you're-wrong-enough-ways-at-once dept.

Why does a car rated for 47mpg fall so far short? The Houston Chronicle features an article on just why EPA gas estimates can be so different from real-world drivers' experience at the pump (or in looking at the dashboard display), in particular for hybrid cars. From the article: "A geometric average of the FTP-75 and HFET results (with city driving weighted at 55 percent and highway driving weighted at 45 percent) produces a vehicle's CAFE fuel economy, which is then incorporated into a manufacturer's corporate average. CAFE is measured using these tests to the present day. In fact, this methodology will be 50 years old when it's used to gauge compliance with the forthcoming 54.5-mpg CAFE requirements in 2025. That kind of continuity is admirable in baseball, but not in transportation. These tests are irrelevant to contemporary real-world driving. For example, the maximum acceleration on either test is 3.3 mph per second. At that rate, it takes more than 18 seconds to hit 60 mph. Even in the horsepower-deprived 1970s, most people were driving harder than that. And the 60-mph maximum speed on the highway test does not accord with the 75-mph truth of today's interstate traffic."

8 of 374 comments (clear)

  1. consistency more important by mondovoja · · Score: 5, Insightful

    Whether those numbers represent a real world mix of driving accurately really doesn't matter all that much, since fuel economy for other driving styles strongly correlates with fuel economy for the conditions that are actually measured. Long term consistency, on the other hand, matters a great deal for car buyers and for evaluating progress on reducing emissions and consumption.

    1. Re:consistency more important by realityimpaired · · Score: 5, Interesting

      Yes, exactly. The CAFE ratings aren't meant to tell you what your personal MPG is going to be, they are meant to tell you how cars of a specific model year compare to each other. If you do 10% better in one car, you'll probably also do 10% better in the other one.

      That may be so, but miles per gallon is a misleading measurement on which to base the "10% better" calculation. 30MPG to 33MPG is *not* 10% better efficiency. In fact, the two are so close that it's within the margin of error for most of us, and can easily be outweighed by simply getting a bad dice roll with the traffic lights.

      If they switched to a burn rate measurement, like L/100km (that the rest of the world uses), or even Gal./100mi, then you actually could do the math in your head for how much more or less efficient the vehicle is. MPG isn't a linear scale, but L/100km is. As a result, the higher the MPG, the less actual benefit you get: 50MPG to 75MPG isn't even close to a 50% improvement in fuel efficiency (it's actually only about 25%), and the disparity between reality and perception only gets worse as you get higher.

    2. Re:consistency more important by nabsltd · · Score: 5, Insightful

      I think that over a certain speed, fuel economy goes way down.

      The smaller the engine, the more you will see this effect. This is because it has to work much harder (relatively) to move the car faster.

      For a larger engine on the same car, you generally get worse gas mileage at lower speeds because the idle fuel usage is higher. But, as you go faster, the larger engine doesn't work much harder, so mileage doesn't drop as much. After a certain speed, a larger engine will be more efficient on the same car.

      This is why turbochargers are now all the rage. You get the advantage of the lower idle consumption of the smaller engine, while still having the acceleration you need for emergencies and the better mileage at higher speeds.

  2. Not only the milage ratings are false by Anonymous Coward · · Score: 5, Interesting

    The biggest reason that real-world fuel economy is so different is that the testing is done with a specific "standard" fuel that does not contain any ethanol or other "oxygenator for cleaner burning fuel". The stochiometric ratio required for proper catalitic converter operation on modern cars is maintained by the oxygen sensor adjusting the amount of fuel injected into the engine - too much oxygen in the exhaust gas, add fuel to decrease; too little oxygen, decrease the amount of fuel. This is a closed-loop system that does not take into account fuels that have additional "oxygenators" added - it only cares about the oxygen in the exhaust gas. Add oxygen from fuel additives, reduce oxygen in the exhaust gas by adding more fuel, reduce mileage. "Clean burning fuels" with additional oxygenators is one of the biggest government-mandated ripoffs ever devised. The "testing" done to prove the "value" of oxygenated fuels is done with a single-cylinder carbureted engine in a test lab, with no emission control systems. In the "bench" testing, a specific amount of fuel is burned with the oxygen in the air, and the resulting exhaust gases analyzed for hydrocarbon emissions. Add an "oxygenated" fuel, burn the same specific amount metered at the same air-fuel ratio, and TADA, look, it burns cleaner! Of course it does - there is now additional oxygen in the exhaust gas! But in the real world, the emission systems on a modern car sees the extra oxygen and adds more fuel to the engine to "correct" the air-fuel ratio and reduce the oxygen level in the output gas. They don't tell that part to congress or the consumer, so the use of "oxygenated" fuel is mandated by the law at both federal and state levels - and so 4.) Profit!

    And the milage you get on the road does not match the testing...

    note: I designed and manufactured fuel control computers for a while, so I know a littile about how things work.

  3. Your mileage may vary... by dtjohnson · · Score: 5, Insightful

    No one cares what the testing procedure is as long as everyone does the same test and it's repeatable. The purpose of the test is to provide a method for consumers to compare different models with respect to their fuel economy, not to provide a precise prediction of exactly what the buyer's fuel economy will be. Everyone drives differently. People warm their car up in the driveway, fill it up with heavy weight, carry lots of passengers, do a lot of long-distance driving, tow trailers, drive up and down hills, ride their brakes, accelerate briskly to beat their neighbor, drive at high altitudes, drive in cold weather, or whatever. Even more significantly, the energy content of 'gasoline' varies widely depending on how much ethanol it has (more is less) and what its boiling point range is. Just do the same test and do it in a way that someone else could repeat the test the same way and get the same result. That's all we need rather it's a 50 year old test or not.

  4. Re:Units wrong by xaxa · · Score: 5, Informative

    For example, the maximum acceleration on either test is 3.3 mph per second.

    It's hard to take a paper seriously when it gets the units of measure wrong.

    What's the problem? That *is* an acceleration.

    (The SI measure is ms^-2, metres per second squared, or metres per second per second. 3.3 (miles/hour)/second = 0.44704 m s^-2.)

  5. OK by JustOK · · Score: 5, Funny

    Could someone explain this with a car analogy?

    --
    rewriting history since 2109
  6. In any case... by BrokenHalo · · Score: 5, Funny

    I was under the impression that the standard unit for fuel consumption in the US is furlongs per hogshead.