A recent study shows that consumers don't have much faith in the mileage ratings on new vehicle window stickers.

With the number of times automakers have had to restate the fuel economy figures for new vehicles, it’s no wonder a recent study finds Americans are skeptical about the mileage numbers slapped on the window stickers of new cars and trucks.

However, the reason for skepticism is a bit surprising: a substantial number of buyers doubt the reported numbers because their personal vehicles outperform the figures on the official fuel-economy labels.

A new AAA survey indicated that a third of Americans do not believe the U.S. Environmental Protection Agency’s (EPA) new vehicle window sticker accurately reflects the fuel economy they achieve when driving.

Consumer advocates have complained for years the EPA fuel economy tests, which are performed in a laboratory-like setting rather than on public streets, don’t accurately reflect the rear-world performance of vehicles. Indeed, EPA itself has cracked down on various automakers, most notably Hyundai, for submitting questionable data. Other automakers, such as Ford, have been forced to restate their fuel-economy claims of certain vehicles.

The mileage ratings for electric vehicles, like the Nissan Leaf and its 99 MPGe rating, still confuses some consumers, according to a recent study.

The situation is further complicated by the fact the fuel-economy numbers of battery electric are calculated using a mathematical formula that translates the battery usage into the fuel-economy numbers of gasoline powered vehicle.

However, AAA analyzed data collected on the EPA’s FuelEconomy.gov website, along with laboratory and real-world vehicle testing, and found that driver behaviors and environmental conditions, rather than vehicle shortcomings, are likely responsible for most fuel economy variances.

“For years, we’ve heard that drivers question whether the fuel economy rating for their vehicle is accurate,” said John Nielsen, AAA’s managing director, Automotive Engineering and Repair. “In the interest of our members, AAA aimed to address this issue with a multi-phase testing series designed to uncover the real reasons behind fuel economy variations.”

AAA engineers conducted a comprehensive analysis of 37,000 records submitted to the EPA, representing over 8,400 vehicle make, model and year combinations, to identify trends in real-world fuel economy. Surprisingly, among the self-reported data, eight out of 10 drivers reported fuel economy that was higher than the combined city and highway EPA mileage rating for their vehicle, according to AAA.

Owners of certain categories of engines, such as powertrains with manual transmissions, diesel and big V8 engines found in trucks, all self-reported better fuel-economy numbers that those automakers reported to the EPA.

For example, owners of vehicles equipped with manual transmissions reported 17% higher fuel economy than EPA ratings. The owners of diesel-fuel vehicles, including light trucks, reported 20% higher fuel economy.

Meanwhile ruck owners with gasoline-fuel V-8 engines reported fuel economy 5% higher than EPA ratings, while owners of turbocharged V-6 engines reported fuel economy that was 9% lower.

“The vast majority of drivers that submit their vehicle’s fuel economy to the EPA report mileage that beats the window sticker rating,” continued Nielsen. “Although self-reported data has limitations, it’s encouraging to see real-world fuel economy that more closely aligns with, or even exceeds, automaker promises.”

In conducting the analysis, AAA engineers identified a list of vehicles that were frequently reported as failing to achieve the EPA’s mileage rating. The majority of these vehicles, including the scrutinized Hyundai and Kia models, have since been retested and, in some cases, mileage ratings were revised. AAA selected three additional vehicles – a 2014 full-size pickup truck, a 2014 large sedan and a 2012 medium sedan – for further testing.

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In partnership with the Automobile Club of Southern California’s Automotive Research Center, AAA tested the vehicles independently to verify the fuel economy. Over the course of several weeks, testing was conducted using a certified dynamometer and on the streets of Southern California. Test results from of all three vehicles confirmed the EPA mileage rating was accurate, leaving AAA to conclude that driving behaviors, vehicle condition, driving environment and terrain are likely responsible for most deviations from EPA ratings that consumers experience.

“In addition to logging hundreds of miles in various driving environments, the research team put the vehicles through EPA-specified testing designed to mimic the real-world conditions, including city, highway and aggressive driving,” said Megan McKernan, manager of the Automobile Club of Southern California’s Automotive Research Center.

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“The findings indicate that while vehicles tested are capable of achieving the EPA rating, a driver’s real-world mileage will vary based on driving style.”

In the next phase of AAA’s fuel economy testing series, to be released in late 2015, researchers will measure the impact that specific driving behaviors, such as acceleration rates and idle time, have on an individual driver’s fuel economy. In the meantime, AAA recommends that drivers take a closer look at their driving habits to understand the role they play in the fuel efficiency of their vehicle.

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“If you drive aggressively, with heavy acceleration, hard braking and driving at higher speeds, your fuel economy is going to suffer,” Nielsen said. “Driving just five miles-per-hour above 50 is like paying an additional 19 cents per gallon for gasoline.”