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Why US Mileage Ratings Are So Inaccurate
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."
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.
The European tests are also flawed, they might be more realistic but the "mileage" is still not applicable to real world situations. The tests are highly optimized, there is almost no way to get these results as an ordinary driver.
There as a consumer program on TV a couple of weeks ago, people were complaining they were only getting 16km/l instead of the advertised 25 km/l for a certain car. This was after driving instructions/coaching from the importer. The conclusion was something like:
Every car is tested in the same way, highly optimized. You will not get these results in real life, but you can use the results to compare cars, a 25 km/l car will be more efficient than a 20 km/l car of the same fuel type for the same driver.
BTW I am able to almost reach the manufactures mileage in my car, but it means I have to drive really slow, stay of the throttle (0-100 km/h in 20s), look ahead/anticipate to avoid breaking/acceleration, drive under the max speed limit, don't drive in the city, don't drive during rush hours, make sure the car is empty (not carrying unnecessary weight). But realistically this will almost never happens.
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.