Feature: What do fuel economy figures mean?

By Jil McIntosh

Styling is sexy and handling is hot, but when it comes to many Canadian buyers, fuel economy is the trump card. Anything involving fuel can usually light up www.Autos.ca’s comment boards faster than a match can take out a gas can. So as a bit of background, here’s a look at fuel consumption from three sides: our site, Natural Resouces’ numbers, and what happens at the pumps.

Fuel consumption in Canada is measured by litres used per 100 kilometres driven, or L/100 km; it’s also measured as Imperial miles per gallon, or m.p.g. That can sometimes cause confusion when looking at American auto sites, which measure in U.S. miles per gallon – and which don’t add up to our numbers, because of a difference in both gallon size and in test methods. We often get complaints from readers, who would prefer that we use the American figures, or report consumption in kilometres per litre. We like to keep our readers happy, but the reality is this: the official measurement for fuel economy in Canada is set by Natural Resources Canada, which uses L/100 km and Imperial m.p.g. The official governing body and the automakers use those figures, and as a result, so do we. If you want, you can switch back and forth with a little math: divide 282.48 by the number of L/100 km to get Imperial m.p.g., or by the m.p.g. to get L/100 km.

You can find all fuel economy figures by visiting Natural Resources Canada’s web site.

There’s also a very handy U.S. site, which explains the improved EPA testing methods, and lets you switch back and forth with the click of a mouse between U.S. miles-per-gallon and L/100 km ratings. You can find it at FuelEconomy.gov; the conversion is found with each vehicle.

So how do Natural Resources Canada and the Canadian auto companies get those numbers? You might be a bit surprised.

As do many people, I thought vehicle testing involved actual driving. On a perfect, cloudless day, the automaker would put the smallest jockey behind the wheel and send him out on a glass-smooth road. In reality, it’s all done in a lab, with each automaker following a method called the Federal Test Procedure, or FTP. The FTP sets out specifics that each car company must follow, including standardized fuels, laboratories, testing equipment, test cycles and calculations.

Each vehicle is run in for about 6,000 kilometres, and then it’s placed on a dynamometer. Only one set of tires turns: if it’s a four-wheel or all-wheel drive model, it’s tested in two-wheel, but with the tests adjusted for the increased weight and engine load of the system. In all cases, a trained driver runs the vehicle on the dynamometer in both a city and highway test.

The city test simulates a 12-kilometre stop-and-go trip, reaching a top speed of 91 km/h but with an average of 32 km/h for the entire test. About four minutes of the test are spent idling, as a car would do at traffic lights. The city test starts with a cold engine – according to Natural Resources Canada, similar to a vehicle that’s been sitting overnight in summer – and then the first eight minutes are repeated when the engine is hot, to simulate a car that was warmed up, driven, and then stopped for a short period of time.

The highway test starts with a hot engine, and while speeds are varied to simulate different kinds of highway and rural roads, the average speed is 77 km/h, with a top speed of 97 km/h. The test takes 13 minutes. According to Natural Resources Canada, fuel consumption values from these test cycles are calculated from the emissions generated, and adjusted to reflect real-world driving conditions.

My test methods are slightly less scientific: I start out in each vehicle with a full tank, and then I drive it for a week. It’s a combination of highway and city driving, and short and long trips – pretty much what the average motorist would expect to do with a vehicle. Then, when I fill it up, I calculate the mileage, based on the kilometres driven and the litres of fuel required to fill the tank again. Usually my numbers fall close to those provided by the automaker, but sometimes I’m way off, which could be caused by a number of reasons – the same factors that you should consider when you’re driving your vehicle, or considering buying a new one and looking at those same figures.

Our test cars here at www.Autos.ca often have poorer fuel economy because they’re relatively new, and haven’t had a chance to “break in”: I’ve often driven vehicles with lower kilometres than the 6,000-km period specified by the official test. While my fuel tests only cover the space of a week – since that’s how long the automaker gives me to road-test its vehicle – keeping track of your mileage longer-term will help to give you a bigger picture of how your car is actually performing. It’s a good idea to monitor your average fuel economy, since some mechanical problems will make themselves known through the vehicle’s increased thirst, such as a malfunctioning oxygen sensor.

What else can affect fuel use? One of the most important factors is how you drive, and it’s entirely possible that you’re not even aware of the consequences of your right foot. I’ve been a passenger with drivers who are actually unaware that their techniques are costing them money at the gas pumps – it’s the way they’ve always driven, and they’ve never noticed their bad habits. These include hard acceleration from a stop, or constantly on and off the throttle instead of maintaining a steady cruising speed. Keep an eye on your speedometer, too. It’s been my experience that many four-cylinder engines are very economical at highway speeds of around 100 km/h, but if you push them to a steady 120 km/h, you can watch the fuel gauge needle move south with surprising haste.

Using the vehicle’s recommended grade of gasoline is important: if you use a lower grade, the car’s sensors will retard the timing to avoid engine damage from pre-ignition. Equally important is tire pressure, since tires that are missing a few pounds of air will need more fuel to turn them. Your tires should be inflated to the recommended pressure that’s found on the label in the door jamb, or in the owner’s manual.

Excess weight will need more fuel, so emptying your trunk of any unnecessary items will help. Be aware that if you’re spending a week as the designated carpool vehicle, your mileage will suffer a little. If you’re always carrying a full load of passengers or cargo, consider that when you buy your vehicle: a smaller engine isn’t going to save you money if it’s constantly straining to move a heavy load that wouldn’t tax the larger engine choice.

When you’re looking at the published fuel economy of new vehicles, it’s best to use them as a comparison tool, rather than as an exact expectation of what you’ll get in the real world. The 9.0 L/100 km vehicle might actually get 11.0 L/100 km when you’re driving it – but that also means that the vehicle rated at 9.5 L/100 km could be expected to get 11.5 L/100 km on the road. And when you’re looking at our test drivers’ real-world figures in their vehicle evaluations, remember that each is driving in different areas, under different conditions, and with different styles. Use both Natural Resources Canada’s numbers and our numbers as a guide, not as the gospel of what you’ll achieve should you buy the car. As I often say, the only fuel figure I know for sure is that I can go a full tank between fill-ups.