I have the I4 auto and max out at 112mph. I know it sounds stupid but if you hit the cruise control it'll stabilize and get about 17MPG. Just for the record, both the I4 5speed auto and v6 6speed auto have TWO overdrive gears.
People say all the time not to keep the fusion at that rate of speed for more then a short time and that not completely true. They are based off tried and (mostly) true mazda engines. You just need to be smart about it. Biggest thing is good tires. MAKE SURE YOU NEVER EVER EXCEED THE TOP SPEED RATINGS OF YOUR TIRES!!! EVER! I had an uncle blow out a tire for that and thank god he survived and is fine today. I'd also recommend good sway bars, and better shocks. The fusion has good ground clearance so make lower it a little. Also if you're going to be going much over 120, get a wing or fin or something so that you car doesn't become a wing and lift from the rear. This is probably why you see so many wagons passing you at 130, they don't have the wing/dome shape to them. I personally don't plan on spending time at those rates of speed, especially since I'm expecting a kid now.
One last word of advice, if you're going to be driving at that speed, I suggest that you take a driving course to teach you how to handle the car at that speed, professionally advice is usually better then someone like me

(just in case, it couldn't hurt and might be fun)