Fuel Economy Doubts: Your Car Won't Get Mileage Posted on the Window It's becoming more apparent that the fuel economy estimates on new-car windows are often very rough estimates. But the knowledge that these numbers are inflated may not necessarily hurt sales. There's no doubt that consumers look closely at fuel economy when determining which car to buy. In a 2012 Consumer Reports survey, mileage was the top consideration for would-be buyers, viewed as more important than a vehicle's quality, value, or safety attributes. The focus - by automakers and buyers alike - on better fuel economy has regularly been causing the average mileage of new cars to inch upward. According to the University of Michigan Transportation Research Institute, new cars sold in August averaged an all-time high 24.9 mpg, compared to around 22 mpg in 2010. Because fuel economy matters so much to buyers, automakers pump up every instance of mileage improvements. Last week, for instance, Honda grabbed bragging rights by announcing the 2014 Accord Hybrid had reached the 50 mpg plateau in city driving, beating out rivals like the Ford Fusion hybrid (47 mpg in the city), Toyota Camry hybrid (43 mpg), and the Hyundai Sonata hybrid (36 mpg). Yet at the same time new-car mileage ratings are rising, more and more skepticism is arising concerning vehicles" ability to measure up in the real world. A couple of years ago,Honda was sued by drivers who said their Civic hybrids weren't getting anywhere near the mileage stated on car windows at dealerships. In late 2012, Kia and Hyundai were forced to lower fuel economy numbers on several vehicle models after an EPA investigation found them to be inflated and unrealistic. Last month, Ford lowered mpg ratings for the C-Max hybrid to 43 mpg after consumer complaints and independent testing showed the model didn't get the 47 mpg promised. MORE: "Your Mileage May Vary": Truer Than Ever with Today's Hybrid Cars Over the weekend the Detroit Free Press published an article investigating why it is that vehicles often do not deliver the fuel economy listed on their official EPA mpg ratings. John O'Dell, senior editor at Edmunds.com, explained that real-world car mileage can vary by 9 mpg from what's printed on a window sticker - perhaps a bit more for hybrids. "The rule of thumb is to take 10% off the sticker for a gasoline engine and 20% off for hybrids," he said. Temperatures and road conditions, one's driving habits, how much weight cars hold when operating, and vehicle maintenance all affect fuel economy. Hybrids are especially sensitive to such factors; C-Max owners came to expect no better than 35 mpg in February because hybrid mpg plummets with thermostat readings. Fuel economy also suffers when drivers are heavy on the gas and brakes, and quite a few drivers are guilty of these sins occasionally, if not regularly. Automakers are understandably worried that continued skepticism about EPA ratings is bad for business. "The industry is cast under one shadow," Bob Carter, senior vice president of Toyota Motor Sales, told the Detroit Free Press. It's not good for the industry to have consumers doubt the information. MORE: Meet the Low-Key, Low-Cost Grocery Chain Being Called "Wal-Mart's Worst Nightmare" However, for now at least, the mini-scandals about inflated mpg ratings don't seem to be hurting sales in any major way. When Kia and Hyundai were forced to lower the fuel economy ratings in late 2012, analysts said the damage to their brands could cost $100 million in lost sales. In light of record sales for Hyundai and Kia last month, the impact doesn't seem quite so grave. Likewise, Automotive News reported that the Ford C-Max sales rose 12% in August, even though that's the month the automaker lowered the vehicle's mpg ratings.