Hello and thank you for your questions
1. You can either write off gas or Mileage. You can not do both. The IRS allows you to choose from Standard Mileage and Actual Expenses. If you choose the standard mileage than you can not switch to Actual Expenses. You have to stay with either/or for the life of the vehicle(meaning the use of the vehicle). The gasoline falls into Actual Expenses. You can find a more detail information at https://ttlc.intuit.com/community/business-expenses/help/can-i-switch-between-the-standard-mileage-r...
2. The miles that you can deduct are the business miles. Personal miles are not deductible. If you have a home office expense, meaning you work out of your home and meet with customers at your home, and then drive to a site than those are business miles. If you are in a business site and drive to a different business site, those are business miles. The way you track the miles is you take the beginning miles from your odometer and when you get to the other site you take the ending miles from your odometer. You need to write the address of location where you started and the address of your destination. You are going to need to write the business purpose too. In case of an audit you will be good.
You can find more information at Topic No511, Business Travel Expenses - https://www.irs.gov/taxtopics/tc511
I hope this answered your questions. Thank you so much and have a wonderful evening
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