Easy. Most fuel efficient speed is 55 mph. (You Canadians and your weird kilometers, I don't get you, lol) So, for you, about 97 km/h? At 55 mph, I can easily get a solid 34 mpg. I've actually been in this scenario before. I swear by the time I filled up, I had to have only had like a quarter of a gallon left. It was that close.