Trip computers are best bet for calculating fuel efficiency

The way many of us calculate our fuel economy is wrong, but there’s an easy way to get accurate figures for how much fuel you use.

I was genuinely surprised when Drew Winter, editor-inchief of Ward’s Auto World magazine, raved to me about the fuel-economy readings from the trip computer in a Chevrolet Cruze diesel he drove from Detroit to Traverse City, 300 miles away in northern Michigan.

“Trip computer” is the auto industry’s name for the electronic readouts that show your average fuel economy, distance to empty and, often, such statistics as average speed and instantaneous fuel economy. The information usually is displayed on the instrument cluster near the odometer.

Trip computers have been around since the 1980s, but the early ones weren’t accurate because the vehicles lacked sophisticated electronics.

I trusted them about as much as the e-mails that promise beautiful Russian women are dying to meet me. No offense, tovarisch. I’m sure you have a lovely dacha.

I calculated my fuel economy the old way, the way my dad taught me in high school, which was probably the way he calculated if a plane could make it over the Himalayas and back to his air base in World War II. I filled the tank, drove, refilled and divided the number of miles by the amount of fuel to refill.

I might as well have been using a World War walkietalkie in an iPhone world.

“The trip computer is absolutely the best way to track fuel economy,” said Wayne Powell, vice president of electronic systems at Toyota’s tech center in Ann Arbor. “It takes that information, compares it to how far you’ve traveled and does the math. It gets a very precise monitoring of the amount of fuel being used.” Experts agree. “We need to know exactly how much fuel is being used by every stroke of every cylinder,” to meet emissions and fuel economy standards, said Roger Clark, senior manager of GM’s Energy Center. “We do a lot of precision measuring of the fuel injectors and exhaust treatment system.” The computer converts that data into fuel economy.

Ward’s relies exclusively on trip computers for its annual list of the industry’s 10 best engines, said Tom Murphy, Ward’s executive editor.

The old method of filling at the pump and dividing miles by gallons also is less reliable than you might expect.

Gas-station pumps are usually accurate — they’re regulated by the states — about how much fuel they deliver, but it’s almost impossible for the average driver to be sure the tank is equally full from one stop to another. That’s because of such variables such as temperature, sloping gasstation parking lots and the flow rates of the pumps.

“The fuel fillers on pumps have lots of variation,” said Frank Markus, technical director of Motor Trend magazine. “It’s very unrealistic to measure fuel consumption based on the pump. Even the same pump can be way off as to what constitutes a full tank from one stop to the next.”

Experts don’t have much faith in smartphone fuel-economy apps either. They’re good calculators, but don’t have direct access to the engine to determine the exact amount of fuel used.