 Diesel fuel C-Tain levels. The C-Tain rating in diesel fuel is 42 to 45. Most diesel engines operate more efficiently with a C-Tain rating of 48 to 50. One treatment of Hot Shot Secret Diesel Extreme will raise your C-Tain seven points, increase fuel economy, and improve cold starts. Hot Shot Secret Diesel Extreme is available nationwide at truck stops, fine farm and auto stores, and online at hotshotsecret.com. Hot Shot Secret.