The Ford Fiesta RX that you see suggested everywhere is trash. It works well for the regular races but in a championship it is useless.
The Honda Civic is an easy win with all upgrades except second engine (stability loss makes the handling very bad). Change gear ratio from -1 to +.8 to make up for the loss in acceleration. You should easily win the championships (silver and gold) with these settings.
Stability is important because the car spins in every other corner if you have it at 10. That's why I removed the second engine upgrade.