Well, I'm sorry to tell you don't have much of a case here. The Canadien gov. gives figures in IMP.GAL or METRIC (Canada is still part of the Commonwealth). The salesman gave you a canadian measure which is 42 Mile per Imp. Gal. Convert this to US gallons, it is about 34.9 MPG. That is close to your 28 MPG you are reading from your car, right? So, the salesman was right. If your car was displaying 38 Imperial MPG instead of 28 US MPG, would you be happy?
Your only case here is why the federal gov. is still using British MPG measures and is not using only metric measures which are less confusing. At the pump, we still buy liters, right?
With some common sense, you should never have thought that the car would do 42 Mile per Us.Gal, that is a hybrid-class fuel consumption that the smallest cars don't even achieve. With the smallest amount of research you would have figured it out.