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I have observed that "it depends".
In my Insight, with the original drivetrain, fuel economy dropped off predictably (in relation to speed) above ~50mph. The faster I went, the more economy I'd lose. Below 50, the economy of steady-state driving would improve, but the car seemed more "sensitive" to outside factors, such as hills, weather, downshifts, needing to accelerate and decelerate, etc. Probably if there were a straight and level road where I could have traveled at 30-35mph in top gear for dozens of miles without any kind of interruption or grade change, economy would have peaked there, but I never really found those roads. In practice, rural roads with a 45-50mph speed limit were peak. In those conditions, the car would reliably deliver 90-100mpg on a warm summer day.
I have observed a similar pattern in my MX-5. Above ~80kph/50mph, economy drops even more sharply than it did in the Insight, likely because the car is less aerodynamic. The fuel economy display is far less granular, but it appears it would get phenomenal economy at lower speeds (it goes into 6th around 50kph/30mph). However, anything less than 80kph roads in-practice result in lower economy, because they tend to have stops, turns, yields, hills, etc. which hurt far more than the lower speed helps, even using good driving technique. The car will get (in freedom units) ~40mpg at 65mph, ~48mpg at 50mph, and might get 50mpg+ even crawling in traffic in 2nd at ~1200rpm, but the moment I need to accelerate and decelerate, that economy tanks, and I'm averaging ~35mpg city.
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