always figured, going 75 in a 65 will maybe shave off 10 minutes... maybe
On a one-hour trip, there's no point. On a four-hour trip (or a three-hour tour) you'll save enough time to actually do something with.
also, for some reason i'm under the impression that the closer i can get to 50 the better my fuel mileage will be.
The more aerodynamic your car is, the less true that is. My '82 300SD gets its best mileage around 75, I think it has something to do with it being very aero (for its day, especially, but pretty good even by modern standards) and maybe something to do with the non-lockup torque converter. AFAICT those are most efficient at the highest RPMs, and that car cruises at pretty high RPM. My '89 240SX was the same, though it had a stick; it was an even more aerodynamic car. And though I don't have personal experience of getting it up over 26 mpg yet, the 1997 A8 Quattro is reputed to get better mileage at 90 than at 75. I don't personally dare go over about 80 in most scenarios, since I don't have all that fancy laser and radar bullshit. It too has a slush box, but it has a lockup TC. It is however more aero than either of the prior examples.
incidentally, when i drive i don't really look at my odometer anymore, i just tend to be extremely comfortable at posted to +5 of posted speeds.
The safest and most efficient thing is to drive with the flow of traffic, but without being worried about maintaining your place in the race, since you're not racing. Most people are most comfortable in such a situation, which is why you typically see long lines of relatively static traffic. I find myself most comfortable when I don't have stuff coming up behind me...