X-Othermic
Either that, or win the allys and get rally cars. Good, and they can be sold for lotsa money.
Yeah, but there isn't a lightweight Rally car for K-car cup, there's definitely no convertible Rally cars for Spider & Roadster cup, and Rally Cars are not trucks so they can't enter the Sport Truck race!
I'd suggest you look here:-
Stock car race comparison/challenge
There's a huge list in post #1 of which vehicles have been used in a totally stock (oil change permitted) configuration to enter & win these events.
The Prizecar Autobianchi Arbarth from Sunday Cup should massacre the Lightweights, any of the trucks (Dodge, Chevy, Ford or Toyota) can easily win the Sport Truck races with no modifications, and you're spoiled for choice on entries for the Spider & Roadster event! You could even take a Chevy SSR truck to win both Sport Truck and Spider & Roadster events!
If you want good A-spec points, the '54 Corvette is a favourite, otherwise take a Viper SRT/10 and leave the AI eating dust (not my idea of fun though.

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Just curious, what have you bought that can't win these events?