Fuel efficiency: two words that loom large in the mind of any car buyer, and a phrase that becomes a personal mantra with every spike in the price of gas.

When people talk about fuel efficiency, what they’re referring to is how quickly a car burns up gasoline. Or, couched another way, it’s a measure of the time between visits to the pump.

The main determinants of a vehicle’s fuel efficiency are its weight and size. “The heavier something is, the more energy is required to move it, so it requires a bigger engine,” says Ron Corbett, a researcher at the Automobile Protection Association.

Another factor is speed. A car can’t be high-performance and fuel-efficient. “If you want more horsepower, you’re going to burn more fuel. There’s no way around that,” says Phil Bailey, a professional mechanical engineer and Montreal-based garage owner.

“That’s why a Formula 1 car does three miles to the gallon.”

Another tradeoff for fuel efficiency, says Corbett, is “refinement.”

“If you have a car with a 1200cc engine, it’s going to be noisier than a mid-size car with a V6,” he says.

Vehicle performance reports in Canada gauge fuel efficiency by how many litres are required to drive an automobile 100 kilometres. (In the U.S., the measurement is miles per gallon.)

They will always cite two statistics: fuel efficiency on city streets and on the highway. A car performs quite differently in each scenario; in most instances, it draws less fuel on the highway.

That’s because a vehicle’s engine is most efficient at a fixed speed; on city streets, you’re constantly depressing the brakes — for stop signs, street lights and cross-walks — and the energy that was propelling the car is constantly being redirected to the braking system, all of which burns more fuel.

The winners
Every year, the
Office of Energy Efficiency (OEE) in the federal Ministry of Natural Resources publicizes its EnerGuide Awards, which provide an index of the most fuel-efficient cars on Canadian roads. (The OEE tests the cars on a fixed course, simulating 20,000 km of driving — 55 percent city and 45 percent highway.)

Not surprisingly,
hybrid cars — which combine a traditional gas motor and an electric one — fare very well. For the 2004 model year, the Honda Insight Hybrid was the most frugal two-seater, notching 3.9 litres per every 100 kilometres in the city and 3.3 L/100 km on the highway.

The thriftiest compact was the Honda Civic Hybrid (4.9 L and 4.6 L, respectively) and the thriftiest mid-size was another hybrid, the Toyota Prius (4.0 / 4.2). Non-hybrid cars on the OEE’s best-of list include the Volkswagen Beetle TDI Diesel (6.2 / 4.6) and the Toyota Echo (6.7 / 5.2).

“If you look at fuel efficiency numbers, it’s almost always a diesel or a small car,” says Bailey. He warns that hybrids are not the be-all of fuel efficiency. Because of the relatively new technology, they’re rather expensive to repair.

“When the public finds out about that, the value is going to drop very rapidly and their second-hand value will be such that your initial investment in fuel saving won’t compare with your loss of money on depreciation,” he says.

Although the Ontario government, for example, grants a $2,000 rebate to consumers who buy environmentally friendly vehicles like the Prius or the Civic Hybrid, each of these cars retails for about $30,000, which is about $10,000 more than a comparable non-hybrid (like the Toyota Corolla, an exceptionally fuel-efficient car in its own right).

Bailey points out that there are 20 cars in Europe that outshine hybrids on fuel consumption. Most of these models, however, are diesel-powered. For the most part, North American manufacturers and consumers shun diesel engines, because they consider them too loud and foul-smelling.

The losers
When it comes to fuel efficiency, most cars are compared to similar-sized vehicles. The Ford Escape, for example, is touted as the most fuel-efficient vehicle in its class (10.3 / 7.8), but that’s only compared to other sport-utility vehicles, which seem insatiable when it comes to fuel.

The most notorious example is the Hummer. Manufactured by General Motors, this erstwhile army truck is spotted with great regularity in hip-hop videos, and more recently on city streets. Not only is it exceptionally bulky (it’s 69 inches wide) but it’s an incredibly gluttonous vehicle, posting a fuel efficiency of somewhere between 15 and 18 L per 100 km.

According to Bailey, a vehicle’s fuel efficiency peaks at about 30,000 or 40,000 kilometres, once the engine has been nicely worked in. But it’s possible to maintain your car’s competence. Take it in for regular tune-ups, monitor the tire pressure and relax the racing impulse.

“Your right foot is by far the biggest factor,” says Bailey. “You don’t need to be the first one away from the traffic light all the time.”

Andre Mayer is a writer in Toronto.