When It Comes to Mileage, Cars Tell the Truth

WardsAuto editors have driven close to 800 cars over 20 years of Ward’s 10 Best Engines testing and written countless engine-related articles during that time. Still, some think my colleagues and I don’t know how to evaluate fuel efficiency.

When we say we use mileage data straight from the vehicle’s trip computer for our logs, there’s a sudden, disdainful look: “Really? I’ve always found my trip computer a bit optimistic when calculating my vehicle’s mileage,” they’ll say with a raised eyebrow and superior tone.

The suggestion is the time-honored method of calculating consumption by measuring gallons burned between fill-ups is the most accurate way to gauge because “everybody knows” automakers program their vehicles to give rosy reports on their secret gas-guzzling.

The engine and combustion experts we have spoken with over the years apparently are part of a vast collusion.

But there are problems with this conspiracy theory that defy logic. First, the rose-colored-glasses (RCG) software, if it exists, works poorly. During our test drives, almost all vehicle trip computers produce numbers that are well below the vehicle’s official Environmental Protection Agency rating. Sometimes the mileage results are downright ugly, thanks to occasional irrational exuberance on our part behind the wheel.

What’s more, the alleged RCG software is especially ineffective in hybrid- and plug-in electric vehicles. In our tests, diesel-powered vehicles usually are the only ones that exceed EPA estimates.

Shouldn’t RCG software be in full cloaking mode in HEVs and PHEVs to prevent the possibility of environmentalist-buyer remorse? Instead, there are lawsuits, especially from hybrid owners, upset about their vehicles not living up to advertised mileage claims, based on observed trip computer numbers. Shouldn’t secret software prevent lawsuits?

Secondly, as my mother used to tell me: “When you start telling lies, you eventually will get caught.”

Trip computers use fuel-efficiency calculations to count down how many miles of range are left in the tank. When does the RCG software start becoming more truthful to prevent drivers from getting stranded? Does it start breaking the bad news gently when it hits the half-empty mark, or keep lying until the engine sputters and dies while indicating 50 miles (80 km) of range is left?

Either way, wouldn’t this electronic chicanery result in millions upon millions of complaints about unreliable fuel gauges?

Last summer, when I raved about the 46.7 mpg (5.0 L/100 km) I clocked on a trip with the Chevy Cruze diesel Detroit Free Press auto critic Mark Phelan gave me the raised-eyebrow treatment when I told him trip computers are the most trustworthy source for vehicle fuel-economy information.

Much to my delight, he checked with a variety of experts at automakers and independent sources such as Brian West, head of the Oak Ridge National Laboratory’s fuels, engines and emissions research center and Frank Markus, technical director of Motor Trend magazine. http://www.freep.com/article/20140202/COL14/302020049/phelan-fuel-economy-trip-computer

They all told him the same thing: Unless you want to spend millions of dollars on highly specialized measuring equipment, a vehicle’s trip computer is the most accurate way to judge its thirst.

“We need to know exactly how much fuel is being used by every stroke of every cylinder,” to meet emissions and fuel-economy standards, Roger Clark, senior manager of the General Motors Energy Center, told him. “We do a lot of precision measuring of the fuel injectors and exhaust treatment system.” The computer converts that data into fuel economy.

That kind of precision easily trumps Grandpa’s method, where temperature, sloping gas station parking lots and variations in the way gas pumps sense when a tank is full all can impact the outcome of fuel-economy measurements.

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