Q to start engine, and E to stop engine. btw, the rocks hitbox is crazy so you might hit it without actually hitting it, lol. Also, the longer you drive the vehicle the more smooth it will be.
I got this idea from UniqueBaconHair, his jailbreak game was awesome and inspired me to try to make a jailbreak game, but I failed, so then I decided to work on the cars. Also, credit to Badimo for making the game Jailbreak! (It's on roblox, also, I do in fact, touch grass.)