DOT numbers work if you do the following for highway and city...
Divide 283 by number of liters/100km highway = A(highway) mpg Canadian
Do same for city.... = B(city) mpg Canadian
Add A and B and divide by 2... = C (EPA averge) mpg Can. if lucky here a rebate is in order
Ordinary (keep up with the trafic style driving) overall mpg = C x .8 mpg (Canadian)
Obviously the result will depend on ratio Hghway/City usage and also vehicle loading and driving style but personal experience over 30,000 km. bears out the resulting number. Bear in mind that there are factors that make a significant difference to the results...head or tail winds....vehicle loading, heavy vehicle is great if destination is lower than starting point...sloping fill-up station pads (can make a significant difference especially from slope to opposite slope (usually not considered and will even out over a long study)....I doubt that all pumps cut at the same fill level...summer temp v winter temp.
US rating seemingly has undergone changes along these lines and their numbers seem to be pretty decent now, when I looked.