"Some people use the MSR camp fuel bottles; there's one size that fits almost perfect into a normal water bottle cage. I have one but gave up using it because it's not enough to avoid having to stop and refuel at gas stations. I tend to wander rural roads avoiding towns, due to the car traffic. With two gallons carried along, I can wander all afternoon--literally hours--without worrying about running out of gas."
Wait... At 150ish mpg, how many miles are you riding. I was under the impression that they got more distance than that.
Scott