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've never been more surprised than when Drew Winter, editor-in-chief 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 little electronic readouts that show your average fuel economy, distance to empty, and often other statistics like average speed and instantaneous fuel economy. The information is usually displayed on the instrument cluster near the odometer.
Trip computers have been around since at least the 1980s, but the early ones were not very accurate, because the vehicles lacked sophisticated electronics. I trusted them about as much as the emails that promise beautiful Russian women are dying to meet me.
I calculated my fuel economy the old way, the way my dad taught me in high school: 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 walkie-talkie 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, Mich. "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."
A wide range of 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.
The old method is also less reliable. 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 variables such as temperature, sloping gas station parking lots and pumps' flow rates.
Experts don't have much faith in smartphones' fuel economy applications either. They're good calculators, but don't have direct access to the engine to determine the exact amount of fuel used.