I think you are partially correct, but there are other factors effecting fuel consumption. The car's aerodynamics (coefficient of drag), the roll efficiency of your tires, and other stuff (I can't think of the other stuff!

)You said that once you reach top gear, cruising at the slowest speed in that gear maximizes mileage...but in my car, my top gear is 6th, and cruising at 4,000 rpms (8,000 rpm red line) means I'm going about 110-115 mph or more. Some problem with your theory!
There is research (no source, I'm too lazy, so take this for what it's worth) indicating that speed differential in traffic is a more important factor in accidents than speed
per se. That is, if traffic is moving at 65-70 mph, and you're going 55 mph, you represent a risk factor. If traffic is going 60 mph, and you're going 75, you represent the risk factor.