There’s the sticker on a new car that tells us what the Environmental Protection Agency (EPA) says our fuel economy should be, and then there’s our car’s actual fuel economy based on our own driving style and conditions. We’ve all come to accept that those two sets of numbers can be vastly different. One emissions testing company is conducting tests based on real-world driving to tell us exactly how much they differ.

In the U.S., fuel economy is measured by the EPA under controlled conditions in a laboratory using a standardized test procedure specified by federal law. For its test, U.K.-based Emissions Analytics equips vehicles with a proprietary “Portable Emissions Measurement System” and drives the vehicles on public roads and highways to replicate the many types of real driving cycles. (For more on the testing procedure, click here.)

The company has tested more than 300 vehicles in the U.K., and in the U.S., it has partnered with Motor Trend to test about 120 vehicles to date. That figure will grow by some 200 vehicles per year, says Nick Molden, founder and CEO of Emissions Analytics.

It might be surprising to learn that Emissions Analytics’ on-road testing revealed an overall variance of only about 1% compared to EPA figures. Nonetheless, some specific models exhibited discrepancies of up to 20%. Click here to see individual vehicle results.

But what makes Emissions Analytics data so interesting is the company’s ability to slice and dice test results in ways the EPA does not. Results from the test, called “Real MPG,” can be analyzed by road type, traffic level, driving style and use of air conditioning, among other variables including fuel additive performance. The conclusions can give individual drivers — and fleets — insights into making more informed vehicle, driving style and routing choices.

Some results back up what we already know. “The most obvious message is, drive slower,” says Molden. The tests indicate that optimal speeds for mpg performance are 45 to 55 miles per hour. Of course, it’s not realistic to always maintain speeds in that range, but a bigger takeaway when looking at speed is the effect of hard acceleration.

Accelerating harder in a vehicle with a smaller engine will more negatively affect fuel economy than vehicles with larger engines, Molden says. “With these super downsized engines, if you drive them exactly as they were designed to be driven, you’ll do well on mpg,” Molden says. “But when you get slightly more aggressive, the mpg falls away more rapidly.”

As the trend to engine downsizing grows in North America, “I would expect to see the gap (between EPA and Real MPG numbers) growing for the smallest engines,” he says.

An aggressive driving style, then, is better suited to a larger engine, which is more forgiving across a range of driving styles.

In looking strictly at road mix, driving 90% in the city versus 90% on highway negatively affected fuel economy by 26% on average of the tested vehicles. Taking road mix and driving style together, Emissions Analytics analyzed the mpg difference between a gentle driver traveling on largely uncongested roads versus an aggressive driver in congested areas. The aggressive driver in congested areas took a fuel economy hit of 11% on average.

When looking at hybrids, the Emissions Analytics tests show that most hybrids return fuel economy figures that are a shade below EPA’s, but not drastically out of line — though there is a “massive spread of performance,” Molden says, between hybrid model types.

And while hybrids perform well in congestion, they’re unusually sensitive to aggressive driving, similar to smaller engines. “If you’re using the battery to get that extra power boost to get away with more aggressive driving, that won’t help your mpg,” he says. “But you can get great results if you nurse it.”

Conversely, the tests show you don’t have to drive a diesel-powered passenger car gingerly to achieve stated fuel economy. “The ‘aggression penalty’ from the diesels we have tested is significantly lower than for gasoline,” Molden says, with the smaller diesels being particularly tolerant of aggressive acceleration.  

Air conditioning affects fuel economy differently for city and highway driving. Using air conditioning in city driving on maximum power knocked 5% off miles per gallon on average. In highway driving, it’s only a 1% hit. That’s because air conditioning is a fixed extra load on the engine, so the percentage of extra fuel burned is lower in highway driving. “If you want to save fuel, open your windows in city driving because the aerodynamic effect is negligible,” Molden says.

Emissions Analytics has also done testing of aftermarket additives — such as those in gasoline formulations as well as off-the-shelf products — to see their effects on fuel economy, with interesting results. “Performance varies greatly by product,” Molden says. “Some work, some don’t.”

Drivers have a choice of fueling up with “top-tier” gasoline from brands such as Chevron, Mobil and Shell, which have proprietary detergent additives, or discount fuels, with minimal additives but still meeting government standards. Emissions Analytics’ tests found that in general the top-tier fuels do, in fact, improve fuel economy.

Molden declined to get into specific brands. But after running cars on a discount fuel and then switching to a top-tier fuel, “It was surprising how noticeable the effect was across a wide range of cars, even after one tank or less than one tank,” he says.

Higher octane fuels also return a fuel economy benefit — though as higher octane fuels have more additives, Molden says it’s hard to prove if the benefit is derived from the higher octane or the additives or some combination. Regardless, the tests show about a 3% to 4% mpg improvement between octane grades of 87 and 89. Of course, from a cost standpoint, the improvement may be negated by the higher price of the higher octane fuel.

If anything, the tests show the importance of spec’ing the vehicle — especially the engine — to driving style, road type and congestion levels. You have more control over your fuel economy than you might think.