In the video you see an old mod, before mods were integrated on the steam workshop.
MaxTCC made a new version, though, you can find it if you search "speedometer mod" on the workshop!
You could also use CustomScenes, which allows for big custom maps in singleplayer, but that's a hassle to install. If you're adamant on getting that, I'd advise visiting the discord: discord.gg/Besiege

What do you use to test your speed?

Then that's why. I am in 0.45a. I never updated because mods didn't work for a while.

Weird. I set the spinning blocks to 0.1 acceleration, they act like sturdier swivel joints if you do that. If you use pre-Multiverse you need to swap them out, but they should work fine in the current version.

Thanks for letting me know, if anyone else has that issue (in the latest, non-experimental besiege version) please let me know and I'll update it to replace the spinning blocks with swivels, and the back wheels with an unpowered one.

I fixed it. I don't know why it worked for your video and not me. I replaced the spinning block closest to the rear with a swivel joint and just as i expected that made it stable.

How do you keep it stable when the engines are on? When I activate the engines it is fast but uncontrollable because of the spinning.

Thanks Ross! I'm glad to see you still active in Besiege. I'm afraid I've been inactive for too long.
Are you going to try your hand at air racing with more than 6 blocks? ;)

Not only does it go fast, it also has plenty of lift for gliding and it controls well at low speeds. Definitely a fun and unique plane!