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By Jil McIntosh
Gas is definitely in the news these days as its price climbs to record levels. It’s always on the minds of the reviewers at Autos.ca – we may not pay for our test cars, but we do pay for the fuel we use – and so, whenever possible, we include information on the fuel consumption we experience.
That can lead to all sorts of emails and reader comments, asking why our mileage is often considerably worse (or, far less frequently, much better) than the official published fuel figures for the vehicle. There are several factors contributing to that, but one major reason is a simple one: we’re human.
Contrary to what you might expect, fuel consumption isn’t gauged on a test track. In fact, the car isn’t really driven at all during the test. It’s all performed in a laboratory, which eliminates variables, and is done by the automakers in procedures that are standardized to government specifications. The results are handed over to Natural Resources Canada, which compiles each year’s fuel figures into its ecoENERGY consumption ratings for the public’s use.
The automaker starts with a vehicle that has about 6,000 kilometres on it for “run-in” and tests it on a dynamometer. A two-cycle test is used – a more realistic five-cycle test, already adopted by the U.S. government, is under consideration in Canada – that simulates city and highway driving. For the city portion, the test represents a 12-kilometre stop-and-go trip, with an average speed of 32 km/h and a top speed of 91 km/h. The 23-minute test includes 18 stops, including four minutes of test time spent idling to represent waiting at traffic lights.
The test is done with a cold engine similar to one that’s been sitting overnight in the summer. The first eight minutes of the test are then repeated with a hot engine start, similar to a vehicle that has been driven, stopped for a short time, and then started up again. The highway trip starts with a hot engine and runs for 31 minutes, covering a simulated 16 kilometres at an average speed of 77 km/h and a top speed of 97 km/h. The speed varies to simulate different highway and rural roads. The test staff doesn’t measure the amount of fuel gone from the tank, but analyzes the exhaust, calculating the fuel consumption from the emissions generated.
Transport Canada occasionally audits the results. Vehicles selected for audit testing include those with advanced technologies that are new or recent to the Canadian market, high-production volume models popular with buyers, and models that haven’t been tested in quite a while. Also subject to audit are those where the automaker reports higher or lower fuel consumption data than expected, or models with previous discrepancies between the reported figures and independent audit test ratings. In this case, the government buys vehicles from dealerships through a bidding process and puts 3,500 km on the odometer using a specific route, specific speeds and a single driver. The fuel is drained and replaced with a standardized test fuel. The vehicle is tested on a dynamometer, where its emissions are analyzed and compared with the figures submitted by the car company.
That’s all the scientific stuff. When I pick up a test vehicle, I make sure it’s full and note the mileage. I then drive it for a week, pretty much as a regular owner would: on the highway and on urban streets, running errands, throwing stuff into the trunk, and keeping up with traffic without seriously bending the law. I try to adjust my driving to the vehicle’s intended purpose, pushing sports cars harder than I do the small grocery-getters. I fill the tank with the recommended fuel grade when returning the car (or as needed throughout the week, if I drive it enough), mark my mileage again, and then figure out the consumption.
To do that, I multiply the number of litres pumped in by 100, and then divide that number by the number of kilometres driven. For example, if I drove 300 kilometres and used 32.5 litres, I’d multiply that by 100 to get 3,250 litres. Divided by the 300 km, it works out to 10.8 L/100 km. To turn that into Imperial miles per gallon – not the U.S. type, which uses a different size gallon – divide 282.48 by the metric fuel number. In this case, 10.8 L/100 km works out to 26 mpg.
Many vehicles now include fuel consumption readouts, both instant and average. I always figure it out by the fuel I pump into the tank whenever I can, but I sometimes only have the computerized number, such as on a single day’s drive on a manufacturer’s launch, when I don’t have the opportunity to use a full tank or to fill the vehicle. In that case, I’ll include it in the story, with an explanation. That said, I’ve sometimes checked a test car’s computerized readout against the “real” numbers I figure out from the fuel used, and have found that the car’s computer is pretty accurate.
The amount of fuel I use in a vehicle is affected by many variables – one of which, I often say, is how much my husband drives the car. Different driving habits can produce considerable differences in fuel consumption, and he admits that I can squeeze much better efficiency out of a vehicle that he can. Even relatively small changes, such as the fact that I take my foot off the throttle marginally sooner than he does when approaching slower traffic, and I accelerate with a gentler touch, add up overall. Ambient temperature, traffic, the type of road, idling time, vehicle speed, the length of the trip, number of passengers and the weight of any cargo can all affect how much fuel I go through in a week.
Fuel figures can also vary across Autos.ca as each writer chalks up individual fuel economy ratings. Living in a relatively flat and temperate part of Ontario, I don’t have to deal with hills as our Vancouver correspondent does, or nastier winters suffered by writers who live further north. The vehicles we drive can also be at various stages of “broken in,” as I’ve driven cars with anywhere from a few hundred to close to 20,000 kilometres on the clock.
Although Canada hasn’t adopted the more real-worldly tests now done south of the border, Natural Resources is gradually building an online fuel consumption calculator that encourages drivers to log in and record their actual fuel consumption on their own vehicles. Once a sufficient database is built up, you’ll be able to look at a specific model and see what kind of real-world figures these cars are racking up.
Of course, just as with the reviews on Autos.ca, there will undoubtedly be all different types of fuel figures, each representative of the driver’s location, vehicle use, the weather, and individual driving habits. In a nutshell, fuel consumption really does come down to YMMV … “your mileage may vary.”
