Autore: ExtremeTech

Toyota Prius hybrid left side

The window sticker says 25 mpg average fuel economy and you’ve never seen better than 22 mpg. What gives? The reasons range from changing technology and how to measure it, to automaker miscues, to journalistic rounding converting the world’s fuel economy nomenclature, to our own driving habits. When it comes to fuel economy, “we’re all victims” translates here to “we’re all culprits.” Here are reasons why the mileage on the Monroney (window) sticker doesn’t match what you get.

EPA can’t keep up with fuel-saving technology testing

Stop us if you’ve heard this one before. Innovative engineers develop new technologies faster than the government can revise its test methods. Start-stop technology, which shuts off at a traffic light and refires when the driver’s foot lifts off the brake, saves 1-2 mpg in city mode testing but the EPA’s tests have in the past just used brief stops, not the 30-60-second traffic lights that urban drivers encounter. For years, automakers omitted start-stop because it added $ 100-$ 200 in costs for a beefier battery and starter motor. Now cars are coming to the US with start-stop that may improve economy beyond what the EPA tests estimate.

On highway testing, the EPA may assume more energy-conscious driving than really exists: top speed of 60 mph, average speed of 48 mph, no air conditioning, and slow acceleration. Cars with better aerodynamics may do better at the higher speeds we drive, so there’ll be a car-to-car variance from true mpg.

EPA testing for fuel efficiency doesn’t reflect the way we actually drive, especially on the highway. The government’s highway test involves a top speed of 60 mph, an average speed of 48.3 mph, no use of heaters or air conditioners and an achingly slow initial acceleration in which it takes more than a minute to go from zero to 50 mph. Blame some of it on the government’s aging lab equipment, such as stationary rollers, being unable until recently to replicate true highway speeds.

Automaker testing errors

Automakers make mistakes in their testing, too. Hyundai for two years crowed that every one of its two most popular models, Sonata and Elantra, got 40 mpg in highway driving, unlike competitors. Late this year Hyundai admitted it made mistakes in dynamometer coast down testing (it simulates real world resistance) affecting 1.1 million 2011-2013 Hyundais and Kias. The Sonata and Elantra were downgraded to 39 mpg highway.

BMW restated the mileage of its 2012 BMW 328i automatic from 36 mpg highway to 33 mpg, and from 24 mpg city to 23 mpg. Automaker mistakes on testing are rare. The previous mpg restatement was in 2001. The Hyundai mpg restatement was big because so much of Hyundai’s advertising was built around hitting 40 mpg rather than price or features.

Mistaking highway for overall mpg

Automakers typically advertise one mileage number, almost always the best one. Heck, always the best one. The EPA ratings include estimated mpg in city driving, highway driving, and a combined number that is 55% city driving. Each combination of engine and transmission is rated separately. The best-selling Honda Accord gets 36 mpg in highway driving with a four-cylinder engine and continuously variable transmission, or 30 mpg combined. But variants of the Accord get as little as 22 mpg combined.

In the past, manual transmission cars got better mileage. Now that’s not always the case. The Ford Focus, one of our 10 Best Tech Cars, is rated 23 mpg city, 32 mpg highway, 26 mpg combined with a manual transmission. With an automatic, it’s 27 mpg city, 36 mpg highway, 31 mpg combined. When you compare cars on fuel economy, make sure it’s an apples to apples comparison. If you see mileage like this, 27-36-31, it’s in this order: city, highway, combined.

User error: We’re not fuel-efficient drivers

Little things we do every day mess up fuel economy: braking late for a traffic light we see far ahead, accelerating quickly to get ahead of the jerk in the next lane, and not checking tire inflation. According to the EPA, for every 3 psi of underinflation, you lose 1% fuel economy, and a lot of cars run with tires underinflated by 10 psi. A serious engine problem such as a bad oxygen sensor can cut fuel economy by as much as 40%. But, says the EPA, a clogged air filter doesn’t affect fuel economy with today’s cars but it does reduce performance. If your car has an Eco setting, you may get gentler acceleration or weaker air conditioning when it’s enabled, but it does improve economy.

Milers per gallon doesn’t work as well as liters per 100 kilometers

Which is better: Upgrading your 15 mpg SUV to a new model that gets 20 mpg, or swapping your 33 mpg subcompact for one that gets 43 mpg? The percentage improvement is greater with the new SUV, 33% vs. 30%. Americans track economy as miles driven per gallon of fuel consumed. Most of the rest of world measures economy as units of fuel consumed for a fixed amount of driving, typically liters per 100 kilometers. It has the advantage of being linear and more easily comparable especially at the extremes (gas guzzlers and hybrids). If a new hybrid jumps from 40 mpg to 43 mpg, Americans may think, “Wow! An extra 3 mpg”), while the rest of the world sees the improvement (lower is better) from 5.9 to 5.5 liters per 100 km — nice but not a reason to dump the old car.

VW XL Qatar

The 20% error: Was that US mpg or imperial mpg?

Read a story online and you have to ask if the writer hails from the UK or Ireland, sometimes Canada, where imperial gallons are cited. They’re roughly 1.2 US gallons. Add in rounding errors if the writer saw a liters-per-100-kilometers rating and converted to the nearest round number, and the amount could be off by 25%. Consider the recent story on Audi and VW working on the 300 mpg car. It’s the L1 concept, meaning a highly efficient car that uses just 1 liter of fuel per 100 kilometers. That translates to 282 imperial mpg, which could be rounded to 300 mpg with only a little bit of fudging. So it’s a 300 mpg car if you use imperial measures and allowing for rounding, a 250 mpg car if you convert to US mpg (based on rounding), or 235 mpg if you directly convert liters per 100 kilometers to US mpg. 235 mpg sounds fantastic, but 300 mpg sounds even more fantastic.

Now read: The 10 best tech cars of 2012