For the 2015 model year, you’re going to feel a new sort of sticker shock when you look at the info sheets pasted in new car windows.

New vehicles are going to get worse fuel economy than the 2014 models, even those that haven’t changed in any way.

Fuel efficiency on the 2015s could fall by as much as 10 to 20 percent over comparable 2014 models. But it’s not the vehicles themselves; rather, it’s the way they’re being tested.

Currently, Natural Resources Canada uses a two-cycle testing program. For 2015, it will implement a five-cycle program, adding three tests that measure fuel efficiency as it’s affected by cold weather, air conditioning, and driving habits. It’s similar to the five-cycle program the U.S. has used since the 2008 model year.

When many people picture fuel economy testing, they figure the automakers wait for the best possible weather, and then send the vehicle out to the track with the smallest, lightest driver they can find behind the wheel. In reality, it’s all done in a lab on a dynamometer, since it’s the only way to ensure the necessary consistency.

The current two-cycle program includes city and highway testing, and if you’ve always thought the published fuel figures sounded unrealistic, you’re right. The city cycle starts with a cold engine and lasts 31 minutes. It covers 17.8 kilometres at an average speed of 34 km/h, with a top speed of 90 km/h, and includes 23 stops. Just 18 percent of the total time is spent idling.

The highway test starts with a warm engine (since it’s assumed that people have to drive at least some distance to get to a highway) and lasts 12 minutes. It reaches an average speed of 78 km/h, with a top speed of 97 km/h.

Both tests are done in an ambient laboratory temperature of 20°C to 30°C. Actual fuel consumption isn’t determined by how much is left in the tank, but by measuring the amount of carbon in the exhaust. Four-wheel-drive vehicles have their systems disconnected and they’re tested in two-wheel, with the results adjusted for the extra weight and engine load of the four-wheel.

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Those two city and highway cycles will still be used, but three others are being added. The new cold test is done the same way as the existing city test, but it’s done in a laboratory that has been cooled to -7°C, since cold weather tends to increase fuel consumption.

The lab is heated to 35°C for the air conditioning test, and the vehicle’s climate control is used to cool its cabin. The test runs nine-and-a-half minutes, covering 5.8 kilometres at an average speed of 35 km/h, and a top speed of 88 km/h. The vehicle stops five times, and idles for 19 percent of the total time. The fifth test is high-speed/quick-acceleration, also run for nine-and-a-half minutes. It has an average speed of 78 km/h and a top speed of 129 km/h, and includes four stops with hard acceleration.

As is done with the two-cycle test, the new tests are conducted by the automakers, using standards set by the government. Natural Resources occasionally audits the results, using vehicles it buys at dealerships, if it suspects the submitted figures aren’t accurate. It can also audit models that haven’t been tested in a long time, or that have new technologies.

New EnerGuide window stickers are on the way, too. For 2015, new-car labels will explain that the numbers are the result of the new testing methods.

There will be even more for 2016, when the labels will be further updated and will generally align with the current U.S. labels. They will include such information as the combined city and highway ratings, CO2 emissions, and the fuel consumption range of other models in that vehicle’s segment. There will also be more information for alternative vehicles such as battery-electric cars and plug-in hybrids.

The 2014 Fuel Consumption Guide, available at Natural Resources, lists vehicles with both the certified two-cycle consumption rating, and also with estimated consumption under the five-cycle rating. This guide, previously available in print at new-car dealerships, is now exclusively found online.

Of course, while the new ratings add some real-world situations, the phrase “your mileage may vary” is still very true. Your fuel economy will be affected by such things as where you drive, how hard you accelerate and brake, if you’re carrying heavy loads, your tire pressure, and how well you maintain your vehicle.

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Top, a 2014 EnerGuide fuel economy sticker; below, a 2015 sticker.

To calculate your mileage, fill your tank completely, and write down your odometer reading. When you need fuel again, fill the tank completely once more, and note how far you’ve driven and how much you put in. Multiply the number of litres by 100, and divide it by how many kilometres you’ve driven. For example, say you use 31.10 litres of fuel to drive 400 kilometres: 31.10 × 100 = 3,110; divide by 400, and you’re getting 7.7 L/100 km.

It’s always best to do this long-term, checking your figures over several tanks of fuel, to get a full picture of how your vehicle’s performing. If you suddenly notice a number that’s considerably out of whack, it could be an indication that your vehicle could use some maintenance.

These upcoming fuel efficiency ratings may be more realistic than they’ve been in the past, but as always, what you really get depends on how heavy or light you are with that right foot.