In an effort to expose the underreported truth behind the government's corporate average fuel economy ratings (CAFE) and to define more accurate fuel economy estimates, the team over at Cars.com has created their own True Mileage Index. The consumer website points out the flaw behind CAFE is that the adjusted calculations misrepresent the numbers posted on new car windows, and the actual fuel economy consumers will realize on the road (e.g., although Honda earned a 2007 CAFE rating of 33.5 mpg, no Honda has a combined mileage of better than 31 mpg). Like the CAFE calculation, the True Mileage Index analyzes a number of factors. However, Cars.com places more weight on actual sales volume, and less on alternative fuels (like E85). Their math places Honda's True Mileage Index at a more realistic 24.9 mpg. In a nutshell, don't get overly excited when you hear that the 2020 CAFE number is going to be 35 mpg -- it's likely to only increase your future vehicle's actual fuel economy by a few mpg.

[Source: Cars.com]