Why I can't go faster hauling another vehicle?

After many issues with my modular 3x3 engine 3 cylinder truck, I managed to make it run and I had no success trying to reduce it's speed so now it can reach 240km/h.

Another problem I found is when I try to pull another vehicle I can't go any faster than 20km/h...

See on the clip I'm full throttle and nothing, and when I disconect the magall it goes crazy but it's faster again.

What I'm doing wrong?, I want to make a truck able to go and rescue another vehicle and it shouldn't be that hard. I'm using a modular engine controller with auto clutch I found on the workshop

EDIT: Ok I added a bunch of weight and that helped a lot, then I maged to set the gearing correct and now I have a reasonable top speed of 95km/h for a truck this size.

Thank you

https://preview.redd.it/7k5omobupsce1.png?width=1409&format=png&auto=webp&s=067732226bb65dcb485f2300f638ef8a7fc21790

https://reddit.com/link/1hzz8pj/video/88qfrdub5nce1/player