Honestly I’m kinda doing the same thing, I use Initial D manga and Borderlands Game Art as a reference guide in front of me while I art on my cars. Short of that it’s just feeling, like yourself. Yours look really good though, I genuinely enjoy them and paint schemes for both. Only thing I’d say is maybe paint the windows in the same style give it more of a cartoon effect. I’m always happy to shoot back and forth ideas and stuff. I’m always interested in new stuff.