Not too many ways to do so. Like @alonely mentioned, SN, gas, gun, CO. Some others could be a bit more gruesome. I believe I read about a guy who beheaded himself with his car, I might have read it here, not sure though. IIRC tied a chain around a tree, tied the chain around his neck and put a brick or something on the gas, which effectively beheaded him.
You could also crash your car at high speeds but I don't recommend this because it runs the risk of harming others, which I am wholeheartedly against. If you're looking for speed, it's hard to beat a gun though. Basically instantaneous, and 99% fatal if you know where to aim.
