Study by International Council of Clean Transportation confirms what we already know.
A new study by the International Council of Clean Transportation (ICCT) looked at fuel economy and CO2 numbers for 2011 model year vehicles and confirmed what we already know: estimates are bunk.
Comparing fuel economy estimates with nearly half a million miles of driving data, ICCT saw a wide variance – sometimes as high as 30 percent – between estimated and actual mileage. They saw a similar variance between CO2 emission numbers.
German automakers seemed to be the furthest away from their fuel economy estimates, with BMW leading the pack at that 30 percent upper bound.
You can find more info at the ICCT From Laboratory to Road website.