I get 16.9 mpg highway driving in my 83 FJ60 with H55f, and keep the rpms around 2500. If I drive faster, like I did when I went to Amarillo a couple of weeks ago from Dallas, my mpg drops significantly. I actually ran out of gas on the Amarillo trip. I was in a hurry, and driving about 75mph, and just didn't want to stop for gas until I got to Amarillo. It's about a 350 mile trip from Dallas. My fuel gauge was dropping fast, and I actually tried to stop in a small town about 25 miles out, but the town had only one gas station, and the pumps were out of order!!! So, I just kept on driving. It started sputtering and died, when I was only about 8 miles away from I-40, where there's at least 50 gas stations! I called my employee who lives in Amarillo, and he came out and we put three gallons of this season's lawn mower gas (he was going to throw it out, as he heard it wasn't any good), and that got me into town for a fillup. I felt kind of stupid. Almost like that Seinfield episode where Kramer was taking a new car for a test drive, and he wanted to see how far below "E" the gauge would go.