Ok, but are you sure a simpler solution isn't simply to fix/re-check the speed being reported? I'm just saying that 100km/h doesn't seem like 100km/h. Not suggesting replication of real-life FoV.
Like this video is less than 100 km/h (around 86-90), but it also feels like that:
If you take a vehicle and ride it through Altis (towns, etc.) you'll see that what you see in the video is what you'll see at 135 km/h in the game. FOV is similar in both, too.
I find that racing games have the same problem, you'd be crawling almost and the game tells you that you're travelling at 100 Km/h...
I'm not sure how they calculate speed anyway, the predator drone apparently reaches 300 km/h and so does the jet, yet the predator drone feels like it's much slower.