A great many factors must be considered to determine mpg. Engine size/make/type, total weight vehicle/rider, aerodynamics, drive system, vehicle speed and terrain.
I get well over a 150 mpg with my 33cc 4 stroke even though total weight is 350 lbs and I live in the mountains where 30% grade hills are common. At best my level ground speed is only 29 mph. What makes my bike unique is the triple chainring shift kit and 7 speed 34-13 freewheel giving me a reduction range of 66.79:1 to 16,25:1. The bike has 9 human/engine driven sequential non redundant ratios.
So regardless of what you desire to use its more so about your engineering skills as to what kind of performance you get.