There's a lot of things that can effect fuel mileage on small motorized bikes, most often it's wind. I've gotten as little as 70 mpg on a four-stroke 100 cc bike ridden into the teeth of a Summer gale here on the coast of Oregon, add in some hills with adverse wind coming down them and you could see as little as 60 mpg. And this is on a motorbike that regularly gets over 100 mpg when ridden in low wind with moderate hills, rides.
Two strokes will get worse. My '75 Honda MT250 Elsinore would normally get about 50 to 65 mpg at about 55 mph. I made a weekend run into Crater Lake to K'Falls from the coast and saw as little as 24 mpg as I was climbing to 7000 feet and fighting a east wind at 20 mph. I filled up in Roseburg and was on the reserve just a few miles from the pass at the Crater Lake rim village. luckily there's a gas station at Fort Klamath or I would have been walking that beast.