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Why MPG Numbers Don’t Add Up

Rick Newman
The Exchange

Your mileage may vary. Seriously.

When car buyers drive a new vehicle off the lot, they usually assume the fuel-economy numbers printed in huge type on the window sticker reflect the mileage they’ll get in actual driving. Er, not really. Several factors conspire to push real-world fuel economy lower than laboratory estimates, and now, the proliferation of new types of powertrain technology seems to be making the miles per gallon calculation even more unreliable.

Ford (F) just lowered the fuel-economy rating on its innovative new C-Max hybrid from 47 MPG in combined city/highway driving to 43 MPG, after the government investigated claims that the company had inflated the mileage numbers. The automaker is also sending C-Max buyers a $550 refund to compensate for the lower fuel economy.

Last year, Korean automakers Kia and Hyundai lowered the mileage claims for several vehicles, including the Kia Soul and Hyundai Elantra, after drivers complained and the government investigated. And a recent evaluation by Consumer Reports found that many models don’t get the advertised fuel economy, especially hybrids and cars with turbocharged four-cylinder engines.

Automakers typically blame overstated mileage on testing errors or procedural snafus. Hmmm. “The automakers are using every tool at their fingertips to maximize the mileage they report,” says John O’Dell of car-research site Edmunds.com.

One reason is that government fuel-economy requirements get tougher every year, and carmakers that don’t meet them must pay fines. With gas prices regularly flirting with $4 per gallon in many areas, consumers also care more about gas mileage than they used to, so automakers work hard to reach MPG levels that will allow them to advertise claims such as 40 MPG.

Several other factors complicate the calculation of MPG numbers. The Environmental Protection Agency, which regulates fuel economy, can’t afford to test the hundreds of vehicle permutations offered every year, so it only spot-checks mileage claims the automakers provide. As a result, “errors” creep through.

The EPA’s testing criteria also date to the days when nearly all cars were powered by gasoline, which is a challenge now that hybrids, plug-ins, diesels and new types of gas-powered cars are becoming more popular. The dawn of electric vehicles like the Tesla Model S, Chevy Volt and Nissan Leaf, for instance, has led the EPA to develop new measures of fuel efficiency such as MPG-equivalent (MPGe) and kilowatt-hours per 100 miles, which aren’t exactly intuitive ways for drivers to compare different types of vehicles.

On top of that, the government has been raising its requirement for the amount of ethanol that must be blended into gasoline, to ratios as high as 10%. That lowers real-world fuel economy even more. Ethanol contains less energy than gasoline and therefore produces lower fuel economy. So if you're pumping ethanol-laced gas into your car (and there's no way to tell) it could cut your mileage by a couple of MPG.

Car buyers who care about MPG should start by discounting the MPG numbers on the window sticker. “As a rule of thumb, take off 10% if it’s a gas engine and 15% if it’s a hybrid,” O’Dell advises. The exception is diesels, which tend to get better real-world mileage than advertised, because diesel burns more efficiently in some scenarios that aren’t captured by the EPA’s testing regime.

One factor affects fuel economy far more than most drivers realize -- their own driving habits. Aggressive driving can easily lower MPG by 30%, and even calm drivers who routinely push 65 or 70 miles per hour on the highway will sharply degrade their fuel economy. Cars tend to be most efficient at around 55 miles per hour, but mileage plunges rapidly the faster you go.

Most drivers can boost their mileage, meanwhile, through gentle driving. That means accelerating slowly, coasting up to stop signs, sticking to the speed limit and maintaining your car properly. If there were more model drivers on the road, in fact, we might not even notice when automakers fudge the numbers.

Rick Newman’s latest book is Rebounders: How Winners Pivot From Setback To Success. Follow him on Twitter: @rickjnewman.