Shiloh is right, listen to him. One mile of 60 mpg plus one mile of 30 mpg means an average of 40 mpg, not 45. Maybe it is easier to realize why by using more extreme figures, for example what would the average mileage be if you did one mile of 10 mpg, followed by one mile of 1 000 000 mpg.

Btw, in the metric system, using l/100km, averaging like this works out nice. So 100 km with 4 l/100km followed by 100 km with 6 l/100km means an average of 5 l/100km.