In Honda's case, the mileage discrepancy can work out to a significantly higher annual fuel bill. At 47 miles per gallon, a typical motorist clocking 15,000 miles a year would expect to burn around 320 gallons annually. At $4 a gallon that would work out to around $1,280. Reduce fuel economy to 40 miles per gallon and the bill rises to about $1,500.
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"The new Accord Hybrid is the most fuel efficient (EPA combined) midsize sedan we've ever made and the most fuel efficient 4-door sedan on the market (EPA-city rating of 50 mpg)," Honda spokesman Steve Kinkade said in a statement Thursday.
"We're very happy to see that in Consumer Reports' testing they confirmed that its 40 mpg average ranks it as the most fuel efficient 4-door midsize sedan they've measured in at least the last 10 years," he said.
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Despite concerns that the industry has been "gaming" the system by tuning vehicles to maximize results in tests, most manufacturers have been sticking by their fuel economy numbers, which are based on testing by the EPA.
—By CNBC Contributor Paul A. Eisenstein. Follow him on Twitter@DetroitBureau or at thedetroitbureau.com.