Don't worry about the speed. It's how you take care of your engine(ie: DMD, sythetic oil changed regularly). I mean I've seen you posting alot around here, and if you read up a little you will see that the Duratecs are designed shitty, and you have to add a little bit to make them run better. For example, my engine too went out and with the 3.0L swap I'm adding the DMD and putting a little wall with holes drilled in it in the oil pan pick up to at least help with the oil starvation. I also plan on running the engine 1/2 a quart over. I mean the little things are what keep the engine going. Out here in Colorado the speed limit is 75, so I really don't think there is a set speed you shouldn't take your car over. It may sound stupid, but I went up to the mountains(a 300 mile drive) with a bad connecting rod 4 times, but since I took care of the engine, it was able to last a long time till the rod blew, which you can see the date in my sig. So basically I would say do the little things to keep your engine running longer.
