You've done your bit — traded in that gas-guzzling SUV for a more fuel-efficient compact car. You've squeezed a few more kilometres out of every litre of gas and are spewing maybe 4,000 kilograms less CO2 into the atmosphere every year.

You're feeling good, but at the same time you're wondering why you're not getting quite the gas mileage that the sticker on the window of your car in the dealer's showroom suggested you might. In fact, you may be falling well short of those lofty goals.

Think of it as another version of the warning in your passenger side mirror: objects may be closer than they appear. Mileage may be greater than what you will achieve.

What do those numbers mean?

They are meant to give you an idea of how economical (or not) a specific vehicle is. Every new car is supposed to carry an EnerGuide fuel consumption label, which will allow consumers to compare the fuel consumption and estimated annual fuel costs of different vehicles.

The label lists how much gas you can expect to burn per 100 kilometres of driving in the city and on the highway. Burn fewer litres per 100 kilometres and you're spending less on fuel and spewing fewer emissions into the atmosphere.

Every new car, van and light duty truck in Canada is supposed to carry an EnerGuide fuel economy label, but there is no law that forces manufacturers to do so. In 1982, Parliament passed The Motor Vehicle Fuel Consumption Standards Act, but never proclaimed it because the auto industry agreed to voluntarily comply with the requirements of the act. Natural Resources Canada says if the vehicle is missing a label, "ask the dealer to see a copy or to show you the fuel consumption ratings for that vehicle in the Fuel Consumption Guide, or ask for the manufacturer's approved fuel consumption rating for the vehicle."

In the United States, all vehicles are required by law to carry fuel consumption stickers.

How are the numbers determined?

In a lab, under better-than-normal conditions.

The vehicle manufacturers do all the emissions and fuel economy tests and they send the results to the appropriate federal department. Environment Canada reviews the emissions summaries while Transport Canada looks at the fuel economy results.

Every year, the federal government buys a specific number of new model vehicles to test them in government labs. The results are compared to the data received from the manufacturers.

Transport Canada's labs use an instrument called a dynamometer, which does not simulate real world conditions, but reflects summer conditions at their best: level paved roads, no wind and just one person in the car.

The numbers are adjusted for "real world" conditions.

Transport Canada sends the results to Natural Resources Canada which publishes a Fuel Consumption Guide every year. You can also compare ratings on its website where you can check fuel economy figures in litres per 100 kilometres or in miles per gallon. You can also see an estimate of how much you can expect on fuel each year and how much CO2 your vehicle will emit every year.

What conditions can affect the actual mileage I will get?

There are many. Driving in cold weather with your heater running will reduce your fuel economy. At 0 C, you will burn eight per cent more gas than on a mild day, but if the temperature's down around -30 C, you'll burn 30 per cent more fuel.

If you drive on soft tires, your wheels are out of alignment and you forget to change the oil, you will burn more gas. Rough asphalt, potholes and gravel-surfaced roads can increase fuel consumption by up to 35 per cent.

Air conditioning will also reduce your fuel economy — but likely not as much as driving on the highway with your windows open.

And if you're into quick starts, fast driving and heavy braking, you will burn more gas than less aggressive drivers.

I'm getting nowhere near the mileage the sticker suggests. Why?

There are many reasons why you may fall short of achieving the mileage on those stickers. Again, the numbers are determined in a lab under better-than-normal driving conditions.

The Canadian Automobile Association and Natural Resources Canada recommend these tips to help improve your gas mileage:

• Keep your engine well tuned.
• Check the engine oil level to be sure the engine is lubricated properly.
• Use regular fuel unless your vehicle manufacturer recommends premium.
• Check your tire pressure regularly, especially after there has been a sharp drop in temperature.
• Turn the car off when it is stopped. Ten seconds of idling uses more fuel than restarting the engine. You only need to idle your car 10 seconds in warm weather and no more 30 seconds in cold weather.
• Aggressive driving saves little time but increases fuel consumption and emissions. Tests show that "jackrabbit" starts and hard braking reduces travel time by only four per cent. However, fuel consumption increases by 39 per cent and some toxic emissions were more than five times higher.
• Remove ski racks and roof racks when not in use. They increase a vehicle's aerodynamic drag.

The U.S. Environmental Protection Agency recently revised its mileage figures, saying the old figures were not reflective of real-world experience. The new figures take effect for the 2008 model year, but the agency has made available a tool that lets people compare old and new ratings for vehicles already on the road.

For instance, under the old rating, a 2007 Honda Civic Hybrid was listed at 49 miles per U.S. gallon in city driving and 51 miles per gallon on the highway. The new figures are more than 10 per cent lower at 40 mpg city and 45 mpg on the highway.

Canadian fuel consumption ratings list the same car at 60 miles per Imperial gallon in the city and 66 on the highway. Imperial gallons are 20 per cent bigger than U.S. gallons — but that still leaves the Canadian rating higher than the old American one.

Similarly, U.S. numbers for the Pontiac G6 drop from 23 mpg in the city and 33 mpg on the highway to 20 mpg in the city and 30 on the highway — a drop of about 10 per cent. In Canada, the same car is listed at 28 mpg in the city and 43 on the highway — higher than the old American rating.

Transport Canada says it's studying the new American system to see if it can be adapted for Canadian driving conditions.

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