I completed almost every offroad speedzone with my s2 street race mc laren. The speed and acceleration is so quick, that it is easy to beat those speed Zones. The only thing i am always doing is to have a good Start, i try entering the Zone with about 280 kmh (Start from far away in the forest), that gives you a good boost for the speed Zone. For a few zones i attachted Rally tires and offroad springs to my mclaren:D
I did all offroad speed,drift and jumps 3 stars even some top 50 with maxed out aventador with rally suspension and rally tyres. I would say worth trying 
All of the advice is good, use it all to help formulate what car and tune you are going to use. The last two Speed Zones were the ones in Byron Bay, both close to each other. I tried a few cars but it was the Rally Fighter with an S2 tune that did it for me. Search “Stunt” in the description. If you want to see someone do it, search YouTube. I have used different cars for the PR Stunts, just whatever felt right to use at the time. If you are on the ground you are going fast, if you’re in the air, you’re slowing down. No jet engines here.
For me, I just need to 3 star all of the Drift Zones and finish PR Stunts. I have my Celica, I have Don Joewon Song’s tune, now I just have to think sideways and start drifting (my cap is on sideways as well, does that help?). Oh and watch YouTube.
If at first you do not succeed, try youtube.
I found it is more about the line you take than the vehicle you use. I was swapping between the Nomad and Aurus.
As suggested get a good run up so you can afford to lose some speed through the bends. Once you find the right lines to take you’ll be able to three star the ones you are struggling with.
Good luck, you can do it.
Git Gud.