120 miles to reserve happens depending on how you filled up. If you fill just to the bottom of the filler neck instead of filling real slow and getting it closer to the top, 120 is normal. If you stand it up straight so it holds more gas, I hit reserve at about 130-140 (Interstate should go farther since it has a bigger tank). Speed has more to do with mileage than anything. If I am running slow in the mountains or like the Blue Ridge Parkway, I get nearly 50 mpg which means I hit reserve at about 210. If I am running a steady 85-95 mph, I get about 27 mpg. I have talked with people who run 130+ steady and hit reserve at 65-70 miles. I usually get about 32-33 mpg.
Reserve should just be a feeling of gradually slowing, slow acceleration, rough running, etc. instead of the engine just stopping. If you are having to coast to the side, switch to reserve earlier. (Forgive me if you know all this)
My worst mileage of ANY trip is always the first tank or two. It just seems to get better as the trip progresses.
