Sight lines, intersections and surface are key factors along with traffic volume. There's one stretch of road about an hour from where I live that has a straight section that is several kms long, has amazing views of the road and off to each side, is fenced back from the road, and has a smooth surface.
Even at the most busy times, you're unlikely to see another car. Go early on a weekend morning, and you can be out there for a half hour or more without seeing another car.
I've tested nearly every vehicle I've owned out there. It's interesting to see how fast fast cars can go, and even regular cars. Even the lowly Corolla got to 160 strongly, but damn, it sure loses steam after that. I think it's gearing and possibly a limiter, as many years ago I took a 1990 Acura Integra along that road and got it to 200, and the Corolla has similar power and probably better aero and couldn't nudge past 180.
The Forester was scary at 180, the max it could see. It didn't like going that fast.
A Mercedes buddy of mine with an uncorked E55 (stupid expensive mod) and I managed to coax it to 300 as per gps. Took every inch of room, but we got it there. That's the fastest I've gone on four wheels.
My C36 was limited to 250 and they weren't kidding. Not one km/h over that. The car was dead stable at that speed. Felt like 100 in most cars. That's something the Germans are good at.
The E300 turbo diesel was also rock steady at its 210 limited speed. The 180-210 trip was slooooow.
Early build Hayabusas lacked speed limiters and would go 320 stock. Later bikes were capped at 300. Either way, they got to 250 in a blink of an eye and then handily to top speed. Uncapped bikes hit a wall built of both air and gearing.
The Miata is great fun to 150, nudging 160 and then downright scary past there. It gets blown around easily and is so loud you feel like you're going 250 at 160.
It needed all the room you could give it, but the lowly C230 could get to its 210 limiter. Like all Mercedes I've had, was stable and easy to drive at speed.
The 190E 2.3-16V was so fun. It pulled like hell past 180, got to 200 and ran flat on its face, crawling to 220 from there.
Old 911 turbo would pull hard in boost in fourth gear until 250, feeling like 300 was easily within reach. Then gearing and air became your enemy and you could see a little past 260, but they car also started to become a handful. The car was beautiful to drive at 220-230, that was a perfect cruising speed.
