Testing and calculation differences
Effective for the 2008 model year, the U.S. Environmental Protection Agency (EPA) has added three additional test cycles to its original two cycles of city and highway testing (See 5-cycle testing). As such, their fuel economy calculations are different in order to incorporate the results of the additional tests. These changes have resulted in differences between the Canadian and U.S. (EPA) numbers. Also note that in Canada, we talk about "fuel consumption" while in the U.S., they refer to "fuel economy".

Correction and adjustment factors
In addition to the testing and calculations differences between Canada and the U.S., the correction and adjustment factors also differ. For example, the U.S. EPA applies a correction factor of 22% to their highway fuel economy test estimates to better approximate real-life driving conditions, whereas in Canada, a 15% correction factor is used.

Therefore, both the U.S. EPA fuel economy ratings and the fuel consumption ratings in the Canadian Fuel Consumption Guide can be considered "correct" as they pertain to vehicles in their respective countries. However, the inherent differences do not allow cross-border comparisons. The ratings are only useful when comparing the fuel efficiency of various vehicles within the country in which you reside.