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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."
Well obviously - it's because your gallons are smaller than proper gallons.
Its all just a game so they can boost there average and still sell the trucks that have terrible MPG that people want.
Memory is deceptive because it is colored by today's events. - Albert Einstein
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.
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.
When I accelerate slowly (yes, I'm the guy in front of you you regularly curse), drive a pickup with a stick shift and a 2.3 liter four-banger, keep my highway speed to about 60 mph (that's about 90 kph for you metric folks), and use my magic powers to keep the headwinds and crosswinds to a reasonable level my little pickup will get what the EPA said it gets: 29 miles per gallon. I think a lot of it really has to do with how a person drives. Now, in practice I drive a lot faster than that but it's nice to know that the EPA actually got it right.
It's really quite a simple choice: Life, Death, or Los Angeles.
When you are calculating the average of ratios (miles per gallon) and the denominator is the thing that changes, you have to use the geometric mean. If they used liters per 100km, then they would use the regular average.
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.
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.)
Could someone explain this with a car analogy?
rewriting history since 2109
German cars now violate physics? Was that a dealer option?
the air resistance is nearly doubled at 75 from 60. Pushing air around actually takes up about 40% of a car's energy at highway speeds. Traveling faster makes the job even harder...The increase is actually exponential, meaning wind resistance rises much more steeply between 70 and 80 mph than it does between 50 and 60.
Do not look at laser with remaining good eye.
I was under the impression that the standard unit for fuel consumption in the US is furlongs per hogshead.
I'm own the much maligned 2013 Fusion Hybrid, and my current tank is averaging about 44 mpg. My work route currently averages between 43 and 50 mpg.
My driving conditions are a mix of heavy suburban traffic and stretch of 25-55 mph interstate, with speeds averaging 15-20 mph during rush our. The terrain is rolling hills, with a delta of about 200 feet.
On a warm (T >70 degree), dry day with no wind and little traffic, the car will easily get the 47 mpg.
Temperature has a large impact on the mpg. The same example above in 25 degree weather will net about 36-38 mpg, consistent with the reporting done over the winter. Obviously, cold starts and running the defroster has a big effect, and the electric traction motor eats away at the battery much quicker at lower temps.
Rain will cut the mpg on my work route to about 43 mpg, and the extra drag is very noticeable. A headwind has the same effect. Tailwinds are fun though, and it kinda feels like sailing when the ICE is off.
Cruising at 55-60 mph on the highway, in no traffic on a warm, dry, and windless day, I can get the 47 mpg.
A quick temperature and mpg plot (assuming dry, windless conditions) looks like:
(T deg F, mpg): (25, 36), (30, 38), (40, 40), (50, 43), (60, 45+), (70, 47+), (80, 45).
There is some roll-off at the higher temps because you have the A/C running.
Driver style has a huge impact on observed mileage, and this cannot be stated enough. My wife is your typical, jackrabbit starting, bumper riding, race-to-red driver. Her mpg is far worse than mine. I doubt she's ever seen 40 mpg. A trip that I can do at 45 mpg, she'll get 36 mpg. I've tried to coach her on the basics of hybrid driving, but she just doesn't get it. I imagine a lot of people are the same way. You either "get" how to drive a hybrid, or you don't.
Consumer reports tends to be a bit sensational. They do get put on a treadmill but the EPA numbers are also based on a circuit of real road driving. 3 tests are done and the average of those tests are then used as the EPA rating.
Additionally theres no such thing as a frictionless treadmill, and the treadmill they use is actually able to adjust its load to simulate real world resistances.
There are plenty of real reasons to bash the EPA ratings, there was no need for consumer reports to exaggerate and make shit up.
First the current 5 cycle EPA test isn't limited to 60mph, it goes up to 80 MPH:
http://www.fueleconomy.gov/feg/fe_test_schedules.shtml
That isn't the real problem. The real problem is that 85% of "EPA Testing" is actually done by the manufacturer themselves. In effect this is a Take home test.
http://www.caranddriver.com/features/the-truth-about-epa-city-highway-mpg-estimates
"While the public mistakenly presumes that this federal agency is hard at work conducting complicated tests on every new model of truck, van, car, and SUV, in reality, just 18 of the EPA’s 17,000 employees work in the automobile-testing department in Ann Arbor, Michigan, examining 200 to 250 vehicles a year, or roughly 15 percent of new models. As to that other 85 percent, the EPA takes automakers at their word—without any testing—accepting submitted results as accurate. "
Since EPA MPG plays a big part in overall advertising campaigns, and potential EPA penalties, there is strong temptation for manufacturers to cheat.
Two years ago Hyundai had an ad campaign featuring how all models of many of it's cars got 40MPG highway without needed special models. Hyundai scored big increase in sales. But later testing a Consumer Reports showed a few of Hyundais models got less than 40 MPG in CR testing. This is ODD because CR testing is more straight forward and the vast majority of cars beat their EPA Highway rating when CR tests them on it's own test. So the CR testing is something of a Sanity check for catching cheaters. Eventually Hyundai was found to have a systemic "mistake" in their testing (AKA cheating). They had to roll back mileage claims across the board and give payouts to customers.
The discrepancy between CR and EPA for Hyundai models before they were caught cheating was 1-3 MPG.
Fords new Hybrids are now falling short by 6-9 MPG and Ford has a new (successful) Ad campaign targeting Toyota, claiming better fuel economy. These new Ford hybrids are the first to make significant sales inroads against Toyota. If anything MPG advertising has even more effect on Hybrid sales.
It isn't hard to see how Fords interests are benefited by high test scores, on a test they administer to themselves, even more than they were for Hyundai before they were caught cheating. It certainly smells like something rotten in Dearborn Mi.
I worked for just over 2 years in a wind tunnel for a company that manufactured cooling equipment (eg, radiators, oil coolers, A/C condensers and evaporators). We tested products for a variety of manufacturers which meant a wide variety of equipment; ie, compressors, farm tractors, semi tractors, passenger cars, and on one occasion a small city bus to be used in Miami, Florida.
We had a reputation for maintaining a very stable, controlled environment (air flow, heat load, dynamometer load, and positioning of thermocouples for sampling temperatures) and consequently consistent test results.
Now in the interest of full disclosure, this was in the early 70's. But at that time, that's also where the manufacturer's typically got their mileage estimates.
I think this might also be the era from where we get the expression "Your Mileage May Vary" (aka YMMV). I think they included this disclaimer in car ads in an attempt to comply with the "truth in advertising" laws (remember those?).
Clearly nobody can drive a vehicle in a manner as controlled as that.
So if the manufacturers are still getting their mileage results from a wind tunnel test, forget it. You'll never match those results especially if you live in a large metropolitan area (where it's not uncommon to sit idling in traffic) or you live in a mountainous area or where you have really cold weather.
There are several really good comments here with additional insight as to why mileage can vary drastically from the manufacturer's estimate; type of fuel mix, for one.
So remember, when you're buying a car and read those mileage estimates, YMMV.
An effective "democracy" creates the illusion the people have a say in their government.
Those unmitigated muscle cars were rated for power on engine dynos with race pipes, no alternator, no drivetrain, etc. There will little standards to horsepower ratings in the 70s. Now, it is still engine power (as opposed to actual power at the wheels), but they are required to be measured with all accessories and emissions equipment.
Plus, brake specific fuel consumption has skyrocketed since then. A 1970s 5.7L engine gets a whopping 200-250 hp, where today's get 400-500.
When I worked at a dyno shop, we were accused of all sorts of mischief when a customers 1960s or 1970s "muscle" car came in and made all of 190 hp. "Your dyno is broken!" Well, the 80s may have produced anemic american cars but the Japanese and Euro cars actually managed, in the late 80s to produce quick vehicles. They also had been using EFI for decades. The Americans produced ridiculously ineffecient, carb'd (until the late effing 90s) pieces of shit until, well, they went bankrupt. Of course then, nobody nuked American manufacturing plants requiring them to build new infrastructure.
Where I live the posted limits are 75 mph. Very few people here drive that slow unless the radar detector is going off or there is a cop actually in view.
The reason I know this is because I *do* drive the posted limit, and I am *constantly* being passed.
Arguing semantics But actually, that isn't valid reasoning. Even if you're constantly being passed, that's not a proof that the number of people passing you is greater than the number of people matching your speed exactly.
Why aren't "official" MPG ratings accurate you ask? Because almost everyone, except stupid consumers, benefits from this system. The politicians can point to rising average fuel economy, real or exaggerated, that burnishes their green credentials. The environmentalists and their pressure groups don't have to admit that fuel economy isn't going up as much as advertised or even worse "declined" from previous inaccurate measurements. The auto companies are also happy with this fiction because it allows them to continue business as usual which is more profitable for them. In short almost nobody cares about accurate "official" MPG numbers because accurate don't serve the interests of anyone with skin in the game. Consumers who care about the real MPG can find this information with a few Google searches or a visit to one of the consumer review sites where they can pay for detailed reports with the real numbers (often worthwhile when researching a major durable goods purchase). What do you expect out of government? Accurate numbers? The truth? Don't be naive.
Additionally theres no such thing as a frictionless treadmill, and the treadmill they use is actually able to adjust its load to simulate real world resistances.
Perhaps nefus was referring to dynamometers, which would provide over-optimistic measurements of fuel efficiency and/or vehicle performance, since they don't account for the effects of aerodynamics on those figures. (I'm not claiming that any organization presents figures obtained this way, merely that that's what nefus may be referring to).
Thank you, Edward Snowden.
"Arguments from authority are worthless." —Carl Sagan
Speaking of power in the gas tank, it seems that all these tests are conducted with straight gas, but all you can find at the pump these days is 10% ethanol, which immediately cuts your mph by 3 to 4 mph.
Very few new car advertising even mentions the difference.
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Entire article is false, EPA changed the testing in 2008. Since 2008 they test mpg up to 80mph and accelerate at 8.5 miles per second, roughly 0-60 in 7 second
So I'm guessing you never bothered to read the article. (I know, what was I thinking).
The new standard is FULLY addressed in the article. Go read it.
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A lot went on in the US automotive world in the 1970's. You started off the with the tail-end of the muscle car era, and by the end of the decade after various government regulations and the fuel crisis scare, you ended the decade with things like the Chevy Chevette, Chrysler K-Cars, and AMC importing the Renaults to the US.. I'd say the horsepower deprived era really started in the mid-late 70's, and went to the mid-80's or so until engine improvements and things like fuel injection and computer controlled timing made it possible to build engines with a decent amount of power and not run into problems with the EPA.
Entire article is false, EPA changed the testing in 2008. Since 2008 they test mpg up to 80mph and accelerate at 8.5 miles per second, roughly 0-60 in 7 second
So I'm guessing you never bothered to read the article. (I know, what was I thinking).
In particular the part where EPA may not actually be doing those additional tests amd simply relying on the manufacturer to estimate those numbers in good faith (aka, fudging it). Similar estimation was also how the EPA revised the numbers for older pre-2008 vehicles that were never subjected to these test.
Because the test course is a fixed length and profile, and they're comparing the number of gallons consumed between vehicles to complete the standardised course.
M/g with fixed M and varying g means that the denominator is changing.
It would be difficult to test the cars on a fair yet realistic basis if you had to drive along some kind of (varying) course until you have consumed exactly one gallon and then measure the distance you have travelled.
Volume / distance is a better metric anyway because it's easier to correctly compare the performance of two vehicles. Your fuel savings suffer from diminishing returns from increasing MPG. An improvement from 10 MPG to 20 MPG (halving your fuel consumption) is much, much, much bigger than an improvement from 40 MPG to 50 MPG (cutting your fuel consumption by only 20%). But an improvement of 1 GPM, or 1 L/100 km, is always going to yield the same amount of savings no matter what your baseline is.
This is particularly relevant when you consider that for most use cases, the amount of travel a particular person needs to do is a fixed variable and the type of vehicle they drive (and hence fuel efficiency) is the independent variable. People choose a car based on their needs; they don't select their commuting route based on the kind of car they drive.
It depends on your criteria for asshole.
I have 15 traffic lights for my 4.4 miles of daily work commute. The traffic engineers have timed all the lights except one such that if you drive the speed limit, you will cruise through all the lights green. When I get behind some "asshole" that will not drive the speed limit, or worse, takes a half mile or more to get up to the speed limit, I can easily double my commute time idling at lights. And each and every light requires a new acceleration.
Unfortunately for me, my town is populated with Civil War widows that can't seem to understand that they are legally permitted to drive at the speed limit or to comprehend the principle of sequenced traffic lights.