Super Woden GP (EP)

Now this requires more effort. A-class cars are the lowest that can reach 300 km/h (like for example NIGHTMARE from the Raven/Japanese cars dealership). They're pretty expensive - all of them cost around 60,000 credits. If you want to be more time effective just avoid buying cars as much as possible while doing career races (I only bought the 2CV from french dealer). It's not hard considering you will get prize cars for winning some races along the way. After getting the car go to FREE RACE, select the FIREWHEEL RING track & go flat out to reach 300 km/h.

As mentioned in the roadmap - you can also go for a quicker method and grind this one out on some low-level single race like Matra Hill or Astral Speedway, but do note that payouts are around 2,500 credits here, so this will make it a boring chore, although a bit faster.