The ratings are different not just because of the gallon size, imperial vs U.S.A. but also as a result of the testing procedures. Prior to 2008 the U.S.A. was using the same testing that Canada is presently using. In 2008 the U.S.A. switched to more realistic testing incorporating; there are 5 tests in the U.S.A: a high speed test, an air conditioning test, and a cold temperature test as well as the previous City course and highway course - with differences.
In Canada, at least until 2015 (talks of changing to U.S.A. model) there are only the two tests, city and highway: The first course the dynamometer mirrors is the city course, which runs for 12 km in 23 minutes of stop-and-go driving at an average speed of 32 km/h. The trip includes 18 stops, with 4 minutes spent idling, and a cold start. The highway course starts on a hot engine and runs for 16 km in 13 minutes, at an average of 77km/h with no stops. The top speed reached is 97 km/h. After the mileage is calculated based on those drive cycles, it is decreased by 10-15% to a more realistic estimate.
In the U.S.A that was the test prior to 2008, however the maximum speed is now 126 km/hr as opposed to 97 km/hr. I can not find the maximum for city driving at this time.
So after 2015 or is it the 2015 model year the mileage claims will change, hopefully.
For the present one should just take the U.S.A. EPA ratings and convert them to Canadian. This is as easy as taking the actual mileage and converting to actual litres. For example 23 mpg is 10.23 L/100 kms. Here is a link for that: