Where do I write off my gas mileage, oil change etc for doordashing

You deduct these expenses as Vehicle expenses on your Schedule C.

Please read this TurboTax article for more information.

See this TurboTax support FAQ for how to enter self-employment expenses -  https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/enter-self-employment-business-expenses-like-home/L1k6HJY4A_US_en_US?uid=m903jkdw