Need for Speed Heat

I made it with a 400+ Porsche 918 Spider with 8,7 Power and 8,8 Speed. You need the biggest Nitro tank (Ultimate, 1x6,8kg) though and you need to find a straight road going downhill. Managed to do it on the final stretch of Dex‘s story event (the 2nd race of 3 where you can win his Skyline).
Engine Parts where all Ultimate (not Ultimate+), rest of Parts a mix of Super, Elite, Elite+.

With Regera or any car around 400 it's easy to do in a straight road.
I recomend try to unlock it on Eden Shores. One of the middle streets going south connect to a pretty large tunnel that ends near a gas station; just find this street, go to where it begins near the northern gas station of this area and hit the pedal going south. Traffic won't be a big issue since it's pretty light in this part of the map and the mentioned street/road has enough space to zigzag through it.

BMW M3 GTR LE '06 6,8 Power 10 Speed

You need 386 km/h please add this to the guide

In the beginning of the game I accidentally chose a wrong character. Deleted my save file to restart and choose another one, then this achievement popped up right after restart.

Consegui essa conquista com o Koenigsegg Regera com:
Virabrequim: Supremo+
ECU: Supremo+
Arrefecimento: Supremo+
Exaustão: Supremo+
Turbo: Supremo+
Nitro: Elite

Chassi
Suspensão: Super
Freio: Elite
Pneu: Elite

Transmissão
Embreagem: Esporte
Câmbio: Esporte
Diferencial: Super

Todas as peças voltadas para o tipo "Estrada"

Sai com o carro do esconderijo do bairro Forte Callahan, peguei a rodovia que corta o mapa horizontalmente e após algumas batidas, consegui atingir os 386,24KM/h ou 240MP/h.

Didnt Mention the speed in KM/H.
DOWNVOTED

I just unlocked this totally by accident/as a glitch. While racing with my friend online, I clipped a cars front end which sent us both into the wall, then I (I don’t know what happened) I broke through the map I guess, he said I just blew past him like nothing. This achievement can be buggy, but in a good way (I was using the Mitsubishi Lancer Evo KS Edition by the way)

For those curious, you should be able to use any car that has a 9.9 power and 10 speed. Obviously the Regera is the easiest option because it's 10 power and 10 speed. I really wanted the Huayra so I bought that, but I didn't realize how difficult hitting 240 would be. I had to go and unlock every Ultimate+ engine part and I barely got enough speed to get the achievement.

I checked as a reference point and my top speed when looking at my car specs was 375 km/h, with power and speed at 9.9 and 10 respectively. Using the 1 x 15 lb Ultimate nos tank I was able to hit 389 km/h and unlock the achievement.

Long story short, it's doable with other vehicles, but you really need to work for the upgrades. Also you can do this in a daytime race so you don't need to worry about NPC traffic. I used a race in Dex's storyline where you drive a loop around the south of the map (called South something in the story but outside of the story I believe it's just called Colombia). Plenty of straights in this one and got it with no hassle once my car was fast enough.

*Edit* Just completed the game and for fun I bought and fully upgraded the Ferrari 488 Pista you can buy for like $250,000 after a 100% completion. For about $400,000-$500,000 you can upgrade it with Ultimate+ parts (if you have them) and it has a top speed of 377, which should also be enough to get this achievement.

I was able to hit using the Audi R8 v10 without much fuss just use nitrous power and it’s yours.