The best configurations for the dirt tracks are all slightly different.
While you're still building up a fleet of cars you should probably start with the ones that will benefit you in multiple tracks. That way you get more points for your money and can hopefully upgrade more cars faster
This guide is still a very useful/relevant resource: RD_'s Racing Guide "2.0"
Based on that I'd recommend you go with one of the following for your next car:
- Lexus LFA (Track/Long ratio/Turbo 3) will serve you well in Speedway, Withdrawal and Convict as well as other tracks like Docks and Uptown.
- Honda NSX (Track/Short ratio/Turbo 2) is good for Commerce, Underdog and Sewage
- Honda NSX (Track/Short ratio/Turbo 3) is good for Industrial, Meltdown and Vector